stream模式下服务端如何异步处理消息？

on_received_messages看起来是个同步函数。如何做到像grpc那样在OnReadDone里处理消息，处理完后自己调用StartRead读取下一条消息？

比如服务端收到消息后需要把消息通过网络转发给另一个服务，如果只能同步处理消息，那么这里就会阻塞线程。如果服务端收到数据后用另一个线程去处理消息，那么就没办法做到反压了。

Start with the stream-mode on_received_messages callback and compare the requested flow with gRPC's OnReadDone and StartRead APIs. Determine whether an asynchronous server-handling design can preserve backpressure while forwarding messages to another service; done would require an agreed implementation scope or API design, which the issue does not currently specify.

Written by the indexing model from the issue text.