Starts the task thread.

def start(self):
        """
        Starts the task thread.
        """
        self._lock.acquire()
        try:
            if not self.is_alive():
                self._thread = threading.Thread(target=self._target, name="raven.AsyncWorker")
                self._thread.setDaemon(True)
                self._thread.start()
                self._thread_for_pid = os.getpid()
        finally:
            self._lock.release()
            atexit.register(self.main_thread_terminated)