Update user destinations handling

Before this change subscribing to a user-specific destination in STOMP
required manually appending a unique queue suffix provided in a header
with the CONNECTED frame.

This change removes the need to do that. Instead STOMP clients can
subscribe to "/user/queue/error" and can then begin to receive messages
sent to "/user/{username}/queue/error" without colliding with any other
user doing the same.

Issue: SPR-11077
Return AbstractSubscribableChannel from @Bean methods

Declare SubscribableChannel @Beans in
WebSocketMessageBrokerConfigurationSupport as
AbstractSubscribableChannel to avoid the need for casting when
registering interceptors.

Issue: SPR-11065
Simplify determination of SockJS path

The SockJS path is now passed to the SockJsService handleRequest method
thus removing the need to guess it.

Issue: SPR-11058