client/authority-discovery: Rework error handling

Instead of `handle_dht_events` returning a `Result<(), Error>`, return
a `Poll<Error>` where `Poll::Pending` signals that there are no more
events to handle and `Poll::Ready(Error)` signals that a fatal error
occured. Non fatal errors are handled within `handle_dht_events`
directly, thus looping in `poll` is not necessary anymore.