Ensuring goodbye packets are sent on shutdown

[kingosticks]

As discussed here librespot-org/librespot#676 (comment) we don't seem to get "goodbye" mDNS messages when stopping the librespot process which leaves the service lingering in clients. I think it's due to this TODO:

libmdns/src/lib.rs

Lines 202 to 207 in 4829c5c

	impl Drop for Shutdown {
	fn drop(&mut self) {
	self.0.send_shutdown();
	// TODO wait for tasks to shutdown
	}
	}

I had a bash at a fix, it is not pretty...