@Override
  public void process(WatchedEvent event) {
    pending.get().process(event);
  }
Placeholder of a watcher which might be triggered before the instance is not yet created.
<p>
{@code ZooKeeper} starts its event thread within its constructor (and that is an anti-pattern),
and the watcher passed to the constructor might be called back by the event thread before you get
the instance of {@code ZooKeeper} from the constructor. If your watcher calls methods of
{@code ZooKeeper}, pass this placeholder to the constructor of the {@code ZooKeeper}, create your
watcher using the instance of {@code ZooKeeper}, and then call the method
{@code PendingWatcher.prepare}.
hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/PendingWatcher.java