This is mostly aiming at future proofness, in case we combine multiple
fetches in a single request. Having an unextracted column would cause it
to be extracted as the first column of the next entity.

This also fixes an assertion when refusing to fetch an entity because of
a failed precondition, since we wouldn't cache the entity.
But moreover, it reduces contention

So far, we only handle the case where a Media has multiple files

Instead, have a FOREIGN KEY in those extension tables
This also mandates a media to be provided when creating a
movie/showepisode, and constify the extension getters

This is now only part of the test implementation, and allows us to
remove quite a bit of overhead

Instead of re-computing it dynamically while it's never going to change

As a result, the Media.Duplicate test is obsolete, since a filename
isn't unique in itself. Only the couple (mrl, folder_id) is UNIQUE.
However, UNIQUE doesn't apply when the folder_id foreign key is NULL
Uniqueness has been manually tested for a real folder/mrl couple

This reverts commit d21a8761.

Using 0 as the default value would cause the VLCMetadataService to
wrongly assume the media was already parsed in case the media gets
loaded from the database (so, if the media isn't parsed during the first
application run)

This is meant to be then plugged with removable devices

Instead of having a per-field SQL request.

Even though it does exactly the same as as executeUpdate/executeDelete,
it improves clarity

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

Instead, have the caller specify which type they want.
This allows us avoid using those ugly std::static_pointer_cast

This is lighter, and allows us to use the interface type instead of
the implementation type as the key() method parameter.

Simply store every media track as a new record in their respective
tables. There is too much incertainty regarding what we will parse and
whether or not the tracks will appear as different for a single media.

This fixes a contraint violation exception when trying to handle
multiple tracks, with the language as their only differences.

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