Fixed getCachedExecutor race condition in MethodReference

Issue: SPR-10884
Remove incorrect package import

Issue: SPR-10880
Polish WebSocket/STOMP Java config

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.
Add Tomcat WebSocket integration tests
Polish WebSocket integration tests
Changed Future<Void> to Future<?>

Changed return values of Future<Void> to Future<?> in AsyncRestTemplate
and AsyncRestOperations.
Fixed method count in AbstractMethodMockingControl's IllegalStateException message

Issue: SPR-10885
(cherry picked from commit 5596154)
Properly handle getMethodDescriptors() null value on IBM JVM

Also internally renaming SimpleNonIndexedPropertyDescriptor to SimplePropertyDescriptor and preferring direct field access wherever possible.

Issue: SPR-10862
(cherry picked from commit 5639aa7)
ThreadPoolTaskScheduler allows for setPoolSize changes at runtime

Also exposing "getPoolSize()" and "getActiveCount()" from ThreadPoolTaskScheduler now, analogous to ThreadPoolTaskExecutor.

Issue: SPR-10883
Javadoc fixes
Polishing
Revised spring-oxm for 4.0

Added "target-package" to the jibx-marshaller element. Also fixed "context-path" and "binding-name" to follow Spring's usual attribute naming convention.

Issue: SPR-10882
Add tests to spring-messaging
Stomp codec

Previously, the broker relay's TCP client used Reactor's built in
delimited codec as part of its parsing of STOMP frames. \0 was used as
the delimiter. This worked for most STOMP frames but, crucially,
not for frames with a body that contained \0: when such a frame was
received it would be truncated.

This commit adds a custom codec that parses STOMP frames more
intelligently. It honours the content-length header allowing it to
correctly parse frames with a body that contains \0. The codec largely
delegates to two new classes: StompEncoder and StompDecoder. For
consistency, code that previously used StompMessageConverter has been
reworked to use these new encoder and decoder classes.