track thumbnail from the first track in an album that has one.
+ rewrite Populate.remove_modified with sets
+ clean the SqliteManager utility class
+ Rewrite ProcessTrackThumbnails to use a process pool instead of a thread pool
+ rewrite track store's remove_tracks_by_filepaths to utilize sets
+ fix: sqlite3.ProgrammingError: Cannot operate on a closed cursor on ProcessAlbumColors()
+ move processing artist images from periodic_scans to Populate
+ bump hash string limit from 7 to 10
+ add last_mod property to database
+ fix: TypeError: '<' not supported between instances of 'int' and 'str' on album page
