Issue: SPR-10703

Issue: SPR-10880

The @Bean method now returns WebSocketHandler allowing it to be
decorated via WebSocketHandlerDecorator.

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").

Ensure configuration provided for WebSocketHandler's (eg interceptors,
or HandshakeHandler) are passed on to the SockJsService if congiured.

Better separate Servlet-specific parts of the configuration to make it
more obvious where non-Servlet alternatives could fit in.

Add more tests.

Improve WebSocket integration tests.

Issue: SPR-10880

Previously, the negotiated version was included in the CONNECTED frame
using the accept-version header. This is incorrect. The version
header should be used.

Issue: SRP-10868

Issue: SPR-10835

Issue: SPR-10866

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.

Upgrade to Reactor snapshot builds to take advantage of TcpClient's
reconnect support that was added post-M1. Now, the system relay session
will try every 5 seconds to establish a connection with the broker, both
when first connecting and in the event of subsequently becoming
disconnected.

A more sophisticated reconnection policy, including back off and
failover to different brokers, is possible with the Reactor API. We may
want to enhance the relay's reconnection policy in the future.

Typically, a broken connection is identified by the failure to forward
a message to the broker. As things stand, the message id then discarded.
Any further messages that are forwarded before the connection's been
re-established are queued for forwarding once the CONNECTED frame's been
received. We may want to consider also queueing the message that failed
to send, however we would then need to consider the possibility of the
message itself being what caused the broker to close the connection
and resending it would simply cause the connection to be closed again.

Components that are using a StompBrokerRelayMessageHandler may want
to know whether or not the broker's unavailable. If they're sending
messages to the relay via an asynchronous channel there's currently
no way for them to find this out.

This commit enhances StompBrokerRelayMessageHandler to publish
application events when the broker's availability changes:
BrokerBecameAvailableEvent and BrokerBecameUnavailableEvent.
Irrespective of the number of relay sessions only a single event is
published for each change in the broker's availability.

Update methods available on WebSocketSession interface.
Introduce DelegatingWebSocketSession interface.

Add SubProtocolHandler to encapsulate the logic for using a
sub-protocol.

A SubProtocolWebSocketHandler is also provided to
delegate to the appropriate SubProtocolHandler based on the
negotiated sub-protocol value at handshake.

StompSubProtocolHandler provides handling for STOMP messages.

Issue: SPR-10786

SI's BeanFactoryChannelResolver was BeanFactoryAware. This commit
brings Spring's BeanFactoryMessageChannelDestinationResolver in line
with the old SI class.

The resolver for /user/{username} prefixed destinations is now
more explicitly designed to store queue suffixes rather than session
id's, which is what we happen to use as queue suffixes.

This allows something other than the sessionId to be used without
having to change many places. It also enables applications to
construct destinations with user-specific queue suffixes without
making assumptions about what's used for queue suffixes. For
example a controller may construct a map with subscription destinations
and send that down to the client.