[musicdb] Make the music database fully dynamic from the scrapers so metadata is always up to date

This allows the music database to everything dynamically from scraped data. What does this mean?

If you have (minimum) a MusicBrainz Album ID set in your ID3 tag and it points to the correct album - and your song IDs match up, everything else (artists, compilation flags, etc, etc) should be fetched from MusicBrainz. So your music should be fully correctly indexed. Data can be kept up to date from Musicbrainz, so you can refresh at any time to get the latest metadata into your library, and artists etc will be updated to match.

This should also fix a bunch of other scanning issues introduced by previous patches. Non-MusicBrainz albums should be left more untouched (just show me my tags please!).