This change adds support for a custom "websocket" scope.

WebSocket-scoped beans may be injected into controllers with message
handling methods as well as channel interceptor registered on the
"inboundClientChannel".

Issue: SPR-11305

Before this change, subscribing to a user destination and use of
@SendToUser annotation required an authenticated user.

This change makes it possible to subscribe to a user destination from
WebSocket sessions without an authenticated user. In such cases the
destination is associated with one session only rather than with a
user (and all their sessions).

It is then also possible to send a message to a user destination
via "/user/{sessionId}/.." rather than "/user/{user}/...".

That means @SendToUser works relying on the session id of the input
message, effectively sending a reply to destination private to the
session.

A key use case for this is handling an exception with an
@MessageExceptionHandler method and sending a reply with @SendToUser.

Issue: SPR-11309

Removal of cached destination is now moved outside the for loop
that removes subscriptions to avoid ConcurrentModificationException.

Also since updateCache is a LinkedHashMap with accessOrder=true,
a simple access with updateCache.get() modify the map.
By iterating over updateCache.entrySet(), we avoid this update.

Issue: SPR-11755

Issue: SPR-11506

Added the ability to send a message only to one user session. Given a
user has two tabs open and the client sends a message to the server
from tab 1, it is now possible to send a reply message to only 1 tab
instead of the default mode of targetting all known user sessions.

Issue: SPR-11506

Issue: SPR-11673

Issue: SPR-11673

Issue: SPR-11673

Issue: SPR-11657

Issue: SPR-11657

DestinationCache is now used for both plain and pattern
destinations. It stores internally the subscriptions map for each
cached destination. Subscriptions are initially created when there
is no cache for the requested destination, and are updated when
subscriptions change.

Issue: SPR-11657

Avoid dependency on order in which expected messages are received.

This change simplifies the implementation of the "test" EventPublisher
and MessageHandler used in the STOMP broker relay integration tests.
The updated implementations use a time-limted poll on a BlockingQueue.

The original fix for SPR-11423:
https://github.com/spring-projects/spring-framework/commit/32e5f57e647022d9ea82c03670936bf31f8467de

was insufficient when using an external broker since the original
destination header has to be in the "native headers" map (i.e. with
STOMP headers) in order to be included in messages broadcast by
the broker.

This commit replaces the "responseDestination" attribute on the
JmsListener annotation by a support of the standard SendTo annotation.

Issue: SPR-11707

This change modifies the names of the Reactor support classes in order
to align with the same changes in the 4.0.x line which now supports
both Reactor 1.1 and 1.0.

Issue: SPR-11636

spring-messaging is full of unused imports. This commit removes all
unused imports from spring-messaging.

Issue: SPR-11653

This commit adds the support of JMS annotated endpoint. Can be
activated both by @EnableJms or <jms:annotation-driven/> and
detects methods of managed beans annotated with @JmsListener,
either directly or through a meta-annotation.

Containers are created and managed under the cover by a registry
at application startup time. Container creation is delegated to a
JmsListenerContainerFactory that is identified by the containerFactory
attribute of the JmsListener annotation. Containers can be
retrieved from the registry using a custom id that can be specified
directly on the annotation.

A "factory-id" attribute is available on the container element of
the XML namespace. When it is present, the configuration defined at
the namespace level is used to build a JmsListenerContainerFactory
that is exposed with the value of the "factory-id" attribute. This can
be used as a smooth migration path for users having listener containers
defined at the namespace level. It is also possible to migrate all
listeners to annotated endpoints and yet keep the
<jms:listener-container> or <jms:jca-listener-container> element to
share the container configuration.

The configuration can be fine-tuned by implementing the
JmsListenerConfigurer interface which gives access to the registrar
used to register endpoints. This includes a programmatic registration
of endpoints in complement to the declarative approach. A default
JmsListenerContainerFactory can also be specified to be used if no
containerFactory has been set on the annotation.

Annotated methods can have flexible method arguments that are similar
to what @MessageMapping provides. In particular, jms listener endpoint
methods can fully use the messaging abstraction, including convenient
header accessors. It is also possible to inject the raw
javax.jms.Message and the Session for more advanced use cases. The
payload can be injected as long as the conversion service is able to
convert it from the original type of the JMS payload. By
default, a DefaultJmsHandlerMethodFactory is used but it can be
configured further to support additional method arguments or to
customize conversion and validation support.

The return type of an annotated method can also be an instance of
Spring's Message abstraction. Instead of just converting the payload,
such response type allows to communicate standard and custom headers.

The JmsHeaderMapper infrastructure from Spring integration has also
been migrated to the Spring framework. SimpleJmsHeaderMapper is based
on SI's DefaultJmsHeaderMapper. The simple implementation maps all
JMS headers so that the generated Message abstraction has all the
information stored in the protocol specific message.

Issue: SPR-9882

An initial commit with expanded support for static resource handling:

- Add ResourceResolver strategy for resolving a request to a Resource
  along with a few implementations.

- Add PublicResourceUrlProvider to get URLs for client-side use.

- Add ResourceUrlEncodingFilter and
  PublicResourceUrlProviderExposingInterceptor along with initial
  MVC Java config support.

Issue: SPR-10933

Issue: SPR-11468

Introduce MessageHeaderInitializer strategy for initializing a
MessageHeaderAccessor.

Add IdTimestampMessageHeaderInitializer implementation that provides
control over ID and timestamp header generation.

Disable ID and timestamp by default in SimpMessageHeaderAccessor and
therefore its sub-class StompHeaderAccessor.

Issue: SPR-11468

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.

AbstractMessageConverter and messaging template implementations now
detect and use mutable headers if passed in.

The SimpMessagingTemplate is optimized to supporting using a single
MessageHeaders instance while preparing a message.

This commit also updates code using the SimpMessagingTemplate to take
advantage of its new capabilities.

Issue: SPR-11468

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

Refine semantics of ID and TIMESTAMP headers provided to protected
MessageHeaders constructor.

Refactor internal implementation of MessageHeaderAccessor.

Support mutating headers from a single thread while a message is being
built (e.g. StompDecoder creating message + then adding session id).

Improve immutablity in NativeMessageHeaderAccessor and in
StompHeaderAccessor.

Optimize object creation for initializing messages and subsequent
accessing their headers.

Introduce MessageHeaderAccessorFactory support to enable applying a
common strategies for ID and TIMESTAMP generation to every message.

Add MessageBuilder shortcut factory method for creating messages from
payload and a full-prepared MessageHeaders instance. Also add
equivalent constructors to GenericMessage and ErrorMessage.

Issue: SPR-11468

Following discussion with Gary.

Issue: SPR-11468

Allow a subclass to modify MessageHeaders as well as override the
strategy for or even skip having `ID` and `TIMESTAMP` headers.

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.

Prior to this commit, @SubscribeMapping mapped methods (backed with
@SendTo* annotations, or not) would send MESSAGEs with the wrong
destination. Instead of using the original SUBSCRIBE destination, it
would use the lookup path computed from the configured prefixes in the
application.

This commit fixes this issue - now @SubscribeMapping MESSAGEs use the
original SUBSCRIBE destination.

Issue: SPR-11648

Issue: SPR-11655

Issue: SPR-11636