Improve shutdown responsiveness of SubProtocolWSH

Proactively notify all active WebSocket sessions when a shutdown is
progress. Sessions then can ignore further attempts to send messages
and also stop stop trying to flush messages right away.
Optimize performance in StompEncoder/Decoder escaping

Issue: SPR-11643
Improve access to WebSocketSession fields

Ensure the Standard- and the JettyWebSocketSession can return the
Principal and accepted WebSocket sub-protocol even after the
session is closed.

Issue: SPR-11621
Fix issue with default executor for message channels

This change ensures correct default settings for message channel's
created through the websocket message-broker XML namespace even
when the channels are customized for other reasons -- e.g. to add
a channel interceptor.

Issue: SPR-11623