try_start_receive是否可以省略_receive_token

Server端连接首次try_start_receive是在RpcListener::on_accept里头调用RpcByteStream::set_socket_connected时出发，另外当发送完成后会在on_read_some里继续触发，虽然每次on_read_some是在不同线程执行，但执行过程是串行的

Client端连接首次try_start_receive是在RpcByteStream::on_connect里，另外发送完成后也是在on_read_some里继续触发，也不会有并行操作的可能

因此在try_start_receive里是不是可以不需要获取token

Start with try_start_receive and trace its callers through RpcListener::on_accept, RpcByteStream::set_socket_connected, RpcByteStream::on_connect, and on_read_some on both server and client paths. Verify the claimed serial execution against the token’s synchronization role; done means a justified decision on whether _receive_token can be removed, with the implementation and concurrency coverage updated if removal is safe.

Written by the indexing model from the issue text.