Support for database / disk synchronization

[denis-stepanov]

An often needed feat is to make sure that AdVent database fully corresponds to a snapshot tree on a disk, or vice versa. For db-djv-pg, the option -o would overwrite the tracks thus ensuring that tracks with the same name but different content are synchronized. What's missing for a complete sync is detection and deletion of stale tracks (i.e. existing at target but not at source). An option -s (sync) could be added to db-djv-pg implying -o and also taking care of deleting stale tracks.