Previously, handling too many librespot events in a row, the spawned hook might not have been finished. In this case, the events would stack up, since the event channel then would not be polled.

By using the tokio::process module, we gain support for waiting for processes asynchronously and even if an event would not be handled directly, it will be handled after the previous hook process has finished.

The mainloop code is simplified by relying on the async / await syntax
instead of implementing all the futures ourselves. In doing so, this
also works, if a different user connects during the playback.

Additionally, the application chooses now between connecting using the
users credentials and starting discovery. If there are valid
credentials, no discovery stream is started at all, which prevents
buggy and surprising behaviour.

Also remove an unneeded clone.

Co-authored-by: Robin van Dijk <robin@robinjint.nl>

This works around the issue, where dbus servers would stay around until
the application ends, although their corresponding session had already
ended. New sessions were then unable to communicate through dbus. Now,
newer servers will just replace any previous name owners.