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

Add accessor for brokerAvailable in AbstractBrokerMessageHandler
Ensure brokerAvailable is set even if eventPublisher is not
Add tests BrokerMessageHandlerTests

Turn off brokerAvailable when StompBrokerRelayMessageHandler stops
Actually stop message handling when brokerAvailable=false
Improve log messages

Issue: SPR-11563

This commit adds an XML namespace equivalent of @EnableWebSocket and
@EnableWebSocketMessageBroker. Those are <websocket:handlers> and
<websocket:message-broker> respectively.

Examples can be found in the test suite.

This commit also alters the way MessageHandler's subscribe to their
respective MessageChannel's of interest. Rather than performing the
subscriptions in configuration code, the message channels are now
passed into MessageHandler's so they can subscribe themselves on
startup.

Issue: SPR-11063

Apply consistent styling to new classes introduced in Spring 4.0.

- Javadoc line wrapping, whitespace and formatting
- General code whitespace
- Consistent Assert.notNull messages

Fix a variety of typos throughout the project, primarily in
comments (javadoc or otherwise) but also in a handful of log messages
and a couple exception messages.

ISSUE: SPR-11123

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.

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.

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.

After this change, annotated message handling methods configured to use
a destination prefix (e.g. "/app") no longer have to include the prefix
in their mapping. For example if a client sends a message to "/app/foo"
the annotated methods should be mapped with @MessageMapping("/foo").

Issue: SPR-10835

Use a single class for the broker availability event with a boolean
that indicates whether or not the broker is available, rather than one
event for an available broker and one event for an unavailable broker.

Publish broker availability events in SimpleBrokerMessageHandler so that
it can be used as a drop-in replacement for
StompBrokerRelayMessageHandler.

Remove base class for STOMP-related message handler classes
(AbstractSimpMessageHandler), polish subclasses and fix issues with
more significant updates to STOMP broker relay.

Introduce base class for SubscribableChannel implementations providing
consistent logging for all channel implementations.

Consolidates new, messaging-related classes from spring-context and
spring-websocket into one module.

The SubscriptionRegistry and implementations are now in a package
together with SimpleBrokerWebMessageHandler and primarily support
with matching subscriptions to messages. Subscriptions can contain
patterns as supported by AntPathMatcher.

StopmWebSocketHandler no longer keeps track of subscriptions and simply
ignores messages without a subscription id, since it has no way of
knowing broker-specific destination semantics for patterns.

SimpleBrokerWebMessageHandler can be used as an alternative to the
StompRelayWebMessageHandler.