With this commit, Jackson builder is now used in spring-websocket
to create the ObjectMapper instance.

It is not possible to use the builder for spring-messaging
and spring-jms since these modules don't have a dependency on
spring-web, thus they now just customize the same features:
 - MapperFeature#DEFAULT_VIEW_INCLUSION is disabled
 - DeserializationFeature#FAIL_ON_UNKNOWN_PROPERTIES is disabled

Issue: SPR-12293

The WebSocketMessageBroker config now allows wrapping the
SubProtocolWebSocketHandler to enable advanced use cases that may
require access to the underlying WebSocketSession.

Issue: SPR-12314

Introduced addScope convenience method on CustomScopeConfigurer (for use in WebSocket configuration)

This change adds collection of stats in key infrastructure components
of the WebSocket message broker config setup and exposes the gathered
information for logging and viewing (e.g. via JMX).

WebSocketMessageBrokerStats is a single class that assembles all
gathered information and by default logs it once every 15 minutes.
Application can also easily expose to JMX through an MBeanExporter.

A new section in the reference documentation provides a summary of
the available information.

Issue: SPR-11739

This change removes the recently added SockJsThreadPoolTaskScheduler
and instead builds support for the removeOnCancelPolicy property in
ThreadPoolTaskScheduler and ScheduledExecutorFactoryBean.

Issue: SPR-11918

Commit 5d2e6f enabled the setRemoveOnCancelPolicy of the SockJS
ScheduledThreadPoolExecutor by default. However that property is only
available in JDK 1.7 or later. This change fixes the issue and
introduces an extension of ThreadPoolTaskScheduler for for use with
SockJS which is necessary in any case since ThreadPoolTaskScheduler
does not expose the underlying ScheduledThreadPoolExecutor otherwise.

Issue: SPR-11918

This change sets the removeOnCancelPolicy on the SockJS
ScheduledThreadPoolExecutor to true. This ensures that cancelled
tasks are removed immediately to avoid the "unbounded retention
of cancelled tasks" that is mentioned in the Javadoc of
ScheduledThreadPoolExecutor:

"By default, such a cancelled task is not automatically removed from
the work queue until its delay elapses. While this enables further
inspection and monitoring, it may also cause unbounded retention of
cancelled tasks. To avoid this, set setRemoveOnCancelPolicy to true,
which causes tasks to be immediately removed from the work queue at
time of cancellation."

Issue: SPR-11918

This change ensures that StompSubProtocolHandler is injected with an
ApplicationEventPublisher for both the Java and XML config.

Issue: SPR-11825

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

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

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

Issue: SPR-11556

Merged web.socket.messaging.config into web.socket.config and introduced web.socket.config.annotation

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

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.