The org.zkoss.zk.ui.util.SessionCleanup interface
A listener could implement SessionCleanup to cleanup a session that is being destroyed.
When ZK Loader is going to destroy a session, it invokes the
cleanup method of this interface such that developers could plug the application-specific codes to cleanup a session.
Instantiation: An independent instance of the given class is instantiated each time before the method is invoked. It means it is thread safe, and all information stored in non-static members will be lost after called.