Add the option to freeze the database during post processing

Add the option to freeze the database during post processing, for adding new artists.

It happens that the post processing step finds albums with new artists that aren't added to Headphones. These albums could be wrongly snatched and downloaded releases, but the post processor will happily post process and add the artists. The check adds an extra option under post processing, and is disabled by default. When enabled, it will check for the artist id or artist name in the artists table. In case a RGID is appended to the folder name, the check will be skipped.

I verified this option briefly, by using a clean database and some downloads in the post processing folder. When the corresponding artists weren't added to Headphones, the albums weren't processed. Then I added some artists, and re-processed. This time, albums and artists were added.