code: protected <T> RequestBuilder doPrepareRequestBuilder(
        ResponseReader responseReader, String methodName, RpcStatsContext statsContext,
        String requestData, AsyncCallback<T> callback) {
  
      RequestBuilder rb = doPrepareRequestBuilderImpl(responseReader, methodName,
          statsContext, requestData, callback);
  
      return rb;
    }
docs: Configures a RequestBuilder to send an RPC request when the RequestBuilder
  is intended to be returned through the asynchronous proxy interface.
  
  @param <T> return type for the AsyncCallback
  @param responseReader instance used to read the return value of the
  invocation
  @param requestData payload that encodes the addressing and arguments of the
  RPC call
  @param callback callback handler
  
  @return a RequestBuilder object that is ready to have its
  {@link RequestBuilder#send()} method invoked.