GUACAMOLE-393: Add session invalidation hooks to UserContext.

guacamole-ext/src/main/java/org/apache/guacamole/net/auth/UserContext.java

@@ -212,4 +212,11 @@ public interface UserContext {
      */
     Collection<Form> getSharingProfileAttributes();
 
+    /**
+     * Invalidates this user context, releasing all associated resources. This
+     * function will be invoked when the user logs out, or when their session
+     * is automatically invalidated.
+     */
+    void invalidate();
+
 }

guacamole-ext/src/main/java/org/apache/guacamole/net/auth/simple/SimpleUserContext.java

@@ -234,4 +234,9 @@ public class SimpleUserContext implements UserContext {
         return Collections.<Form>emptyList();
     }
 
+    @Override
+    public void invalidate() {
+        // Nothing to invalidate
+    }
+
 }