Stops the task thread. Synchronous!

def stop(self, timeout=None):
        """
        Stops the task thread. Synchronous!
        """
        self._lock.acquire()
        try:
            if self._thread:
                self._queue.put_nowait(self._terminator)
                self._thread.join(timeout=timeout)
                self._thread = None
        finally:
            self._lock.release()