synchronized byte[] get() throws InterruptedException {
      while (!delete && data == null) {
        wait();
      }
      byte[] d = data;
      data = null;
      return d;
    }
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