code: public static long delayResponse() {
      Call call = CurCall.get();
      long res = 0;
      if (call != null) {
        call.delayResponse();
        res = delayedRpcId.getAndIncrement();
        delayedCalls.put(res, call);
      }
      return res;
    }
docs: If invoked from the RPC handling code will mark this call as
  delayed response. It returns the id of the delayed call.
  The response will only be sent once sendDelayedResponse method
  is called with the id returned from this one.
  @return id of the delayed response.