Since we're caching some entities, we ended up with a media containing
an album track that contains an album that contains the original media,
thus leading none of those shared pointers to be released

Reading from the DB is "cheap", and the cache hurts the maintainability
of the code.
This removes the CachePolicy and CacheColumn field from TablePolicy.
Instead, a PrimaryKeyColumn has been added to TablePolicy

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 removes the old std::string artist property on AlbumTrack

This will avoid random crash due to dangling pointer in case the DB was
released

So we don't have to call .c_str() every 10 lines

This allow the Cache helper to set it automatically, and avoid cache
misses when relying on the primary key as the cache column (since the
primary key was always 0)