synchronized void set(byte[] data) {
      this.data = data;
      notifyAll();
    }
Used to store the newest data which we want to sync to client zk.
<p/>
For meta location, since we may reduce the replica number, so here we add a {@code delete} flag
to tell the updater delete the znode on client zk and quit.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/zksyncer/ClientZKSyncer.java