Allow use of @SendToUser even w/o authenticated user

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

spring-websocket/src/test/resources/log4j.properties

@@ -5,6 +5,5 @@ log4j.appender.console.layout.ConversionPattern=%d{HH:mm:ss,SSS} [%c] - %m%n
 log4j.rootCategory=WARN, console
 log4j.logger.org.springframework.web=DEBUG
 log4j.logger.org.springframework.web.socket=DEBUG
-
-#log4j.logger.org.springframework.web.socket=TRACE
+log4j.logger.org.springframework.messaging=DEBUG