When fetching the device of a network folder, we might need to wait a
bit before the device is discovered.
It not, we will just bail out

Drop AlbumArtistRelation table and only use the AlbumTrack table.
This reduces the database size, simplifies the code, and increase the
similarity between unittests & actual metadata parsing handling

That will spawn a query with its dedicated count request. This will help
fixing some requests with aggregation.
It also can improve performances when the listing request uses many
joins that aren't required when counting the results

Since res was copied, it was quite unlikely to change from within the
predicate lambda

If the playlist insertion task came before, we could drop it after
inserting the media, but without inserting the media in the playlist