@Override
  public synchronized void close() {
    closed = true;
    queue.clear();
    if (resumer != null) {
      resumer.resume();
      resumer = null;
    }
    notifyAll();
  }
A ResultScanner which will only send request to RS when there are no cached results when calling
next, just like the ResultScanner in the old time. Mainly used for writing UTs, that we can
control when to send request to RS. The default ResultScanner implementation will fetch in
background.
hbase-server/src/test/java/org/apache/hadoop/hbase/client/ScanPerNextResultScanner.java