use a thread name for the event loop

[siddontang]

Hi @stepancheg

Now you use thread::spawn to start the event loop thread, it is not convenient to know the thread CPU takes with top -H. It is better to pass a customized thread name in config, or we can use grpc-server-loop or grpc-client-loop by default.

If you think it is fine, we can send you a PR.

[stepancheg]

Thank you for the suggestion!