Add XML namespace for WebSocket config

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