GitLab

We now only process one file at a time, as we do for snapshots.
The correct way would be to spawn more threads from the Parser class,
shall we want to process multiple items at once

This removes the old std::string artist property on AlbumTrack

It's now true when all metadata services have been run. The type is
still exposed and can be used to figure out if the basic metadata are
available

Processing is done by a threadpool (limited to a single thread so far)
This will allow using asynchronous modules one after another
Fixing VLC service cleaup code