The BufferingStompDecoder now decorates rather than extend
StompDecoder. This allows a single StompDecoder instance to be
configured and extended independantly while buffering remains a
separate concern.

Mutate rather than re-create headers when decoding STOMP messages
before a message is sent on a message channel.

Use MessageBuilder.createMessage to ensure the fully prepared
MessageHeaders is used directly MessageHeaderAccessor instance.

Issue: SPR-11468

The UserDestinationMessageHandler adds a header providing a hint for
what the original destination a user may have used when subscribing.
That is then used when writing messages back to WebSocket clients to
ensure they dont see the internally used, transformed user destination.

This change moves the header name constatn to make it more broadly
applicable. For example SPR-11645.

Issue: SPR-11655

Issue: SPR-11578

Issue: SPR-11575

Issue: SPR-11578

Issue: SPR-11527

Issue: SPR-11527

BufferingStompDecoder message buffer size limit can now be configured
with JavaConfig MessageBrokerRegistry.setMessageBufferSizeLimit() or
with XML <websocket:message-brocker message-buffer-size="">.

Issue: SPR-11527

Before this change the StompDecoder decoded and returned only the first
Message in the ByteBuffer passed to it. So to obtain all messages from
the buffer, one had to loop passing the same buffer in until no more
complete STOMP frames could be decoded.

This chage modifies StompDecoder to return List<Message> after
exhaustively decoding all available STOMP frames from the input buffer.
Also an overloaded decode method allows passing in Map that will be
populated with any headers successfully parsed, which is useful for
"peeking" at the "content-length" header.

This change also adds a BufferingStompDecoder sub-class which buffers
any content left in the input buffer after parsing one or more STOMP
frames. This sub-class can also deal with fragmented messages,
re-assembling them and parsing as a whole message.

Issue: SPR-11527

Before this change the decorator ensured that for a specific WebSocket
session only one thread at a time can send a message. Other threads
attempting to send would have their messages buffered and each time
that occurs, a check is also made to see if the buffer limit has been
reached or the send time limit has been exceeded and if so the session
is closed.

This change adds further protection to ensure only one thread at a time
can perform the session limit checks and attempt to close the session.
Furthermore if the session has timed out and become unresponsive,
attempts to close it may block yet another thread. Taking this into
consideration this change also ensures that state associated with the
session is cleaned first before an attempt is made to close the session.

Issue: SPR-11450

Since we now wrap the WebSocketSession with a concurrent decorator, the
synchronized keyword around message sending needed to be removed.

Issue: SPR-11586

This change exposes the WebSocketSession attributes through a message header.
The StompSubProtocolHandler adds this to incoming messages.
For now messaging handling  methods can access the map via @Header, e.g.:

@Header(StompHeaderAccessor.SESSION_ATTRIBUTES) Map<String, Object> attrs) {

Issue: SPR-11566

Before this change, when a client subscribed to a "user" destination
(e.g. /user/foo), actual messages received in response to that
subscription contained the server-translated, unique user destination
(e.g. /foo-user123).

This is not an issue for clients such as stomp.js since the
subscription is unique and sufficient to match subscription responses.
However, other STOMP clients do additional checks on the destination
of the subscription and the response.

This change ensures that messages sent to clients on user destionations
always contain a destination that matches the one on the original
subscription.

Issue: SPR-11423

Issue: SPR-11327

The interface is to be implemented in addition to
java.security.Principal when Principal.getName() is not globally unique
enough for use in user destinations.

Issue: SPR-11327

Issue: SPR-11277

Introduced SockJsSession interface and moved SockJsSessionFactory and SockJsServiceConfig to sockjs.transport; added initialize(SockJsServiceConfig) method to TransportHandler interface; extracted TransportHandlingSockJsService from DefaultSockJsService; moved sockjs.support.frame to sockjs.frame and extracted (Default)SockJsFrameFormat from SockJsFrame; moved SockJsHttpRequestHandler to sockjs.support; removed Jackson 1.x support

Two concurrent threads should not send a message on a single WebSocket
session at the same time, for example see:
http://docs.oracle.com/javaee/7/api/javax/websocket/RemoteEndpoint.Basic.html

In StompSubProtocolHandler it is quite possible that multiple messages
may be broadcast to a single WebSocket client concurrently.

This change adds synchronization around the sending of a message to a
specific cilent session.

Issue: SPR-11023

Fix remaining Java compiler warnings, mainly around missing
generics or deprecated code.

Also add the `-Werror` compiler option to ensure that any future
warnings will fail the build.

Issue: SPR-11064

Before this change spring-messaging contained a few WebSocket-related
classes including WebSocket sub-protocol support for STOMP as well
as @EnableWebSocketMessageBroker and related configuration classes.

After this change those classes are located in the spring-websocket
module under org.springframework.web.socket.messaging.

This means the following classes in application configuration must
have their packages updated:

org.springframework.web.socket.messaging.config.EnableWebSocketMessageBroker
org.springframework.web.socket.messaging.config.StompEndpointRegistry
org.springframework.web.socket.messaging.config.WebSocketMessageBrokerConfigurer

MessageBrokerConfigurer has been renamed to MessageBrokerRegistry and
is also located in the above package.

Before this change the amount of logging was too little or too much
with TRACE turned on. This change separates useful debugging
information and logs it under DEBUG and leaves more detailed
information to be logged under TRACE.

Before this change subscribing to a user-specific destination in STOMP
required manually appending a unique queue suffix provided in a header
with the CONNECTED frame.

This change removes the need to do that. Instead STOMP clients can
subscribe to "/user/queue/error" and can then begin to receive messages
sent to "/user/{username}/queue/error" without colliding with any other
user doing the same.

Issue: SPR-11077

Issue: SPR-10930

Fix error in te code that handles the result of sending a heartbeat

Fix error in processing DISCONNECTED frames that closed the TCP
connection before the message was sent.

 - Polish javadoc for CONNECTED_USER_HEADER
 - Improve method ordering

Previously, handling of a STOMP CONNECT message and sending of a
CONNECTED response was performed by StompProtocolHandler if it was
backed by SimpleBrokerMessageHandler, or left up to the real message
broker if it was backed by StompBrokerRelayMessageHandler. This
wasn't ideal as it should be StompProtocolHandler's job to simply map
messages to and from the STOMP protocol, not to do part of the
broker's job and respond directly to CONNECT.

This commit introduces a new message type, CONNECT_ACK. When it
receives a CONNECT message, SimpleBrokerMessageHandler will now
respond with a CONNECT_ACK message that StompProtocolHandler can map
into a STOMP CONNECTED message. The CONNECT_ACK message contains the
CONNECT message as a header so that StompProtocolHandler has access to
its accept-version header.

StompProtocolHandler has been simplified so that a CONNECT message
is always passed to the output channel, irrespective of whether it's
backed by a simple broker or a real broker. The handleConnect flag,
and the code that would set it correctly depending on the app's
configuration, has been removed.

Previously, the STOMP broker relay did not support heart-beats. It sent
0,0 in the heart-beats header for its own CONNECTED message, and set the
heart-beats header to 0,0 when it was forwarding a CONNECTED from from a
client to the broker.

The broker relay now supports heart-beats for the system relay session.
It will send heart-beats at the send interval that's been negotiated
with the broker and will also expect to receive heart-beats at the
receive interval that's been negotiated with the broker. The receive
interval is multiplied by a factor of three to satisfy the STOMP spec's
suggestion of lenience and ActiveMQ 5.8.0's heart-beat behaviour (see
AMQ-4710).

The broker relay also supports heart-beats between clients and the
broker. For any given client's relay session, any heart-beats received
from the client are forwarded on to the broker and any heart-beats
received from the broker are sent back to the client.

Internally, a heart-beat is represented as a Message with a byte array
payload containing the single byte of new line ('\n') character and
'empty' headers. SubscriptionMethodReturnValueHandler has been updated
to default the message type to SimpMessageType.MESSAGE. This eases
the distinction between a heartbeat and a message that's been created
from a return value from application code.

Before this change, the StompProtocolHandler always responded to
clients with a CONNECTED frame, while the STOMP broker relay
independantly forwarded the client CONNECT to the broker and waited
for the CONNECTED frame back. That meant the relay had to buffer
client messages until it received the CONNECTED response from
the message broker.

This change ensures that clients wait for a CONNECTED frame from
the message broker. The broker relay forwards the CONNECT frame to
the broker. The broker responds with a CONNECTED frame, which the
relay then forwards to the client. As a result, a (well-written)
client will not send any messages to the relay until the connection
to the broker is fully established.

The StompProtcolHandler can now be configured whether to send CONNECTED
frame back. By default that is off. So when using the simple broker,
the StompProtocolHandler can still respond with CONNECTED frames.

The relay's handling of a connection being dropped has also been
improved. When a connection for a client relay session is dropped
an ERROR frame will be sent back to the client. If a connection is
closed as part of a DISCONNECT frame being sent, no ERROR frame
is sent back to the client. When the connection for the system relay
session is dropped, an event is published indicating that the broker
is unavailable. Reactor's TcpClient will then attempt to re-restablish
the connection.

Previously, the broker relay's TCP client used Reactor's built in
delimited codec as part of its parsing of STOMP frames. \0 was used as
the delimiter. This worked for most STOMP frames but, crucially,
not for frames with a body that contained \0: when such a frame was
received it would be truncated.

This commit adds a custom codec that parses STOMP frames more
intelligently. It honours the content-length header allowing it to
correctly parse frames with a body that contains \0. The codec largely
delegates to two new classes: StompEncoder and StompDecoder. For
consistency, code that previously used StompMessageConverter has been
reworked to use these new encoder and decoder classes.

Issue: SPR-10818

Issue: SPR-10703