GUACAMOLE-393: Add session invalidation hooks to UserContext.

guacamole/src/main/java/org/apache/guacamole/GuacamoleSession.java

@@ -252,6 +252,10 @@ public class GuacamoleSession {
             }
         }
 
+        // Invalidate all user contextx
+        for (UserContext userContext : userContexts)
+            userContext.invalidate();
+
         // Invalidate the authenticated user object
         authenticatedUser.invalidate();