ScheduledExecutorService may not be shutdown in ChannelFactory #619

public class ChannelFactory implements AutoCloseable {
    private final ScheduledExecutorService recycler = Executors.newSingleThreadScheduledExecutor();
     ....
     public void close() {
     ....
      if (certContext != null) {
        recycler.shutdown();                                      // recycler would not shutdown when TLS is disable
        if (certWatcher != null) {
          certWatcher.close();
        }
      }
    }
}