Fix synchronization issue in StompSubProtocolHandler

Two concurrent threads should not send a message on a single WebSocket
session at the same time, for example see:
http://docs.oracle.com/javaee/7/api/javax/websocket/RemoteEndpoint.Basic.html

In StompSubProtocolHandler it is quite possible that multiple messages
may be broadcast to a single WebSocket client concurrently.

This change adds synchronization around the sending of a message to a
specific cilent session.

Issue: SPR-11023
Add ability to customize message channels

@EnableWebSocketMessageBroker message channel configuration can now be
customized via WebSocketMessageBrokerConfigurer. It is necessary to
make this easy and even required as part of the basic configuration
since by default the message channels are backed by a thread pool of
size 1, not suitable for production use.

Issue: SPR-11023