@Override
  public synchronized void sendResponseIfReady() throws IOException {
    // set param null to reduce memory pressure
    this.param = null;
    connection.doRespond(this);
  }
If we have a response, and delay is not set, then respond immediately. Otherwise, do not
respond to client. This is called by the RPC code in the context of the Handler thread.
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/NettyServerCall.java