Add ImmutableMessageChannelInterceptor

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