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

spring-websocket/src/test/java/org/springframework/web/socket/support/TestPrincipal.java

@@ -20,7 +20,8 @@ import java.security.Principal;
 
 
 /**
- * An implementation of Prinicipal for testing.
+ * An implementation of {@link Principal} for testing.
+ *
  * @author Rossen Stoyanchev
  */
 public class TestPrincipal implements Principal {