'Shutdown the torrent health checker.
Once shut down it can\'t be started again.
:returns A deferred that will fire once the shutdown has completed.'

def shutdown(self):

    self._should_stop = True
    self.cancel_all_pending_tasks()
    for tracker_url in self._session_list.keys():
        for session in self._session_list[tracker_url]:
            self.session_stop_defer_list.append(session.cleanup())
    defer_stop_list = DeferredList(self.session_stop_defer_list)
    self._session_list = None
    self._torrent_db = None
    self.tribler_session = None
    return defer_stop_list
