Before this change, subscribing to a user destination and use of
@SendToUser annotation required an authenticated user.
This change makes it possible to subscribe to a user destination from
WebSocket sessions without an authenticated user. In such cases the
destination is associated with one session only rather than with a
user (and all their sessions).
It is then also possible to send a message to a user destination
via "/user/{sessionId}/.." rather than "/user/{user}/...".
That means @SendToUser works relying on the session id of the input
message, effectively sending a reply to destination private to the
session.
A key use case for this is handling an exception with an
@MessageExceptionHandler method and sending a reply with @SendToUser.
Issue: SPR-11309
97fb308b6b