Default executor doesn't wait for status updates to be ack'd before shutting down

  void _shutdown()
  {
    const Duration duration = Seconds(1);

    LOG(INFO) << "Terminating after " << duration;

    // TODO(qianzhang): Remove this hack since the executor now receives
    // acknowledgements for status updates. The executor can terminate
    // after it receives an ACK for a terminal status update.
    os::sleep(duration);
    terminate(self());
  }