Fix shutdown not actually shutting down

[Naatan]

Ran into this issue: #27

[cuu508]

Would it be possible to also wait for the in-flight SMTP sessions to complete?

Server.Shutdown() in the http packages does this:

Shutdown gracefully shuts down the server without interrupting any active connections. Shutdown works by first closing all open listeners, then closing all idle connections, and then waiting indefinitely for connections to return to idle and then shut down. If the provided context expires before the shutdown is complete, Shutdown returns the context's error, otherwise it returns any error returned from closing the Server's underlying Listener(s).