Improve third-party fetch logic

Currently, we only make one attempt at fetching third-party data for a track or topmusic entity.

This is potentially problematic if the user is experiencing connection issues at the moment of fetch. We should only update the database on an empty fetch response if it can be ensured that this was not due to the local connection. Otherwise, it's possible that the user could be indefinitely barred from third party data for the given items.

We should also schedule retries on fetches.