Threadactive

fix: in Agent.tick -> self.stop -> self.stop_backend

1. _Active -> _Backend
2. done_message -> done, abort_message -> abort, clear_message -> clear
3. Agent: start -> start_backend, stop -> stop_backend

1. -_Active._agent
2. stop agent when backend thread exit
3. fix wrapper return value

+ clear_message

+ break the thread if handle_message return False

+ done_message & abort_message