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This was causing the insertion assert to trigger, due a thread querying
the database while:
- Holding the cache lock
- Acquiring the database lock
- Inserting the fetched entity in cache if it didn't exist

While another thread was inserting an entity while:
- Holding the database lock
- Releasing it after the insertion was done
- Holding the cache lock
- Checking that the entity didn't exist in cache

Since the locking order wasn't consistent, the 1st thread was
interupting the 2nd before it was done with the entire insertion
method, causing an inconsistent state.

Instead of the function name, display the fully decorated methods &
line, when available

Media now always have a single artist
Artist are being added media
The album artist meta is used to generate the list of artists
The artist meta isn't used for artists list

And fallback to AlbumArtist if no artist is provided

And make it an enum class

...since it's unused.

How could this build with g++ ?!

This makes it easier to scan a row and extract values to variables,
without having to manually pass the column index.
In the likely event we add/remove columns, this will help prevent
regressions as well.

This might need to be reverted when properly addressing files deletion
(while monitoring filesystem or when reloading)
However, as of now, I'm not confident this respect the object ownership
graph anyway

This is mostly a workaround before a proper architectural fix