Imp: handle request from server #26
Description
At present, the rpc means that only the server can receive request from client and then send back its response. However, the client can not handle the request from the server.
To handle request from server, the client should register its callback. what's more, we have an agreement that the sequence in the request package from the client should be an odd number and the sequence in the request package from server is an even number.