docs: private ReceiveMessageFuture issueFuture(int size,
                                               QueueBufferCallback<ReceiveMessageRequest, ReceiveMessageResult> callback) {
          synchronized (futures) {
              ReceiveMessageFuture theFuture = new ReceiveMessageFuture(callback, size);
              futures.addLast(theFuture);
              return theFuture;
          }
      }