Option to gracefully stop the server/check if the server is still running

[LukasKalbertodt]

Currently the server, once started, just runs forever and there is not really a way to know whether it's still running. So it might be useful to add a way to gracefully stop the server, similar to how hyper does it. As far as I understand it is already possible to stop the server by using tokio::select! or something like that, but that might just drop ongoing HTTP connections. We might also want to add a method to Controller to check whether the server is still running.