API server stop

[garyyu]

Close #1934

[garyyu]

A confusing problem:

		Server::bind(&addr)
		.serve(router)
		.with_graceful_shutdown(rx)

with_graceful_shutdown works in Grin binary, but can't work in test_p2p, function return OK but API port not listening. No idea about the root cause of this.

[hashmap]

that's where I stopped last time, it also works fine in API tests, I have a loop there to start/stop server twice, works like a charm

[garyyu]

Close for now, since no good solution to fix the
strange problem in #1943 (comment).