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