fix d2ClientBuilder shutdown corner case

[brycezhongqing]

Fix a corner case bug in d2ClientBuilder shutdown logic.

Corner case: when _config._executorService == null

  if (_config._executorService == null)
    {
      LOG.warn("No executor service passed as argument. Pass it for " +
        "enhanced monitoring and to have better control over the executor.");
      _config._executorService =
        Executors.newSingleThreadScheduledExecutor(new NamedThreadFactory("D2 PropertyEventExecutor"));
      executorsToShutDown.add(_config._executorService);
    }

Then _executorService will be added to executorsToShutDown. Finally, when d2Client call the shutdown method, the _executorService may be closed twice. Like

1. first place
2. second place

[bohhyang]

should the first place be removed then? if the executors are shut down in the second place. But I guess it's ok as long as the second place has the isShutdown check guard.

[bohhyang]

don't need warning, info level is fine.

[brycezhongqing]

should the first place be removed then? if the executors are shut down in the second place. But I guess it's ok as long as the second place has the isShutdown check guard.

Sadly. No. As executorsToShutDown.add(_config._executorService); will only execute when if (_config._executorService == null)

If _config._executorService != null, the first place should be executed