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

Issues: SPR-12283

This commit introduces a new OriginHandshakeInterceptor. It filters
Origin header value against a list of allowed origins.

AbstractSockJsService as been modified to:
 - Reject CORS requests with forbidden origins
 - Disable transport types that does not support CORS when an origin
   check is required
 - Use the Origin request header value instead of "*" for
   Access-Control-Allow-Origin response header value
   (mandatory when  Access-Control-Allow-Credentials=true)
 - Return CORS header only if the request contains an Origin header

It is possible to configure easily this behavior thanks to JavaConfig API
WebSocketHandlerRegistration#addAllowedOrigins(String...) and
StompWebSocketEndpointRegistration#addAllowedOrigins(String...).
It is also possible to configure it using the websocket XML namespace.

Please notice that this commit does not change the default behavior:
cross origin requests are still enabled by default.

Issues: SPR-12226

This change adds a ChannelInterceptor that flips the immutable flag on
messages being sent. This allows components sending messages to leave
the message mutable for interceptors to further apply modifications
before the message is sent (and exposed to concurrency).

The interceptor is automatically added with the STOMP/WebSocket Java
and XML config and the StompSubProtocolHandler leaves parsed incoming
messages mutable so they can be further modified before being sent.

Issue: SPR-12321

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

Issue: SPR-12314

This commit updates the default location of the SockJS' client library.
The previous location is being retired by the project maintainers.

The new default location is backed by several CDN providers:
* https://cdn.jsdelivr.net/sockjs/0.3.4/sockjs.min.js

See sockjs/sockjs-client#198

Issue: SPR-12254

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

Issue: SPR-12249

The WebSocket messaging namespace now exposes configuration options for
custom argument resolvers and return value handlers.

Issue: SPR-12217

Issue: SPR-12091

Issue: SPR-11845

Issue: SPR-11845

Issue: SPR-11660

In order to be able to use separators like "." (used by default
by most broker relays) instead of "/" for destination patterns
handling, the PathMatcher used in spring-messaging can now
be customized easily thanks to XML websocket namespace
or JavaConfig.

AntPathMatcher has been updated in order to use the configured path
separator instead of an hardcoded "/" for path concatenation.
Extension handling is now disabled when the "." separator is configured.

Issue: SPR-11660

Issue: SPR-11961

This change removes most logging at INFO level and also ensures the
amount of information logged at DEBUG level is useful, brief, and
not duplicated.

Also added is custom logging for STOMP frames to ensure very readable
and consise output.

Issue: SPR-11934

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

Issue: SPR-117711

Issue: SPR-11766

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.