Add -c shortarg for --checksum-every, and increase default checksum time from 7 days to 14 days.
Significantly clarify and cleanup media_check.py logging.
Add --prune to control pruning of missing files (when enabled, prune _all_ files immediately instead of lazy-pruning); improve granularity of --maximum-media-verifications enforcement (only count actual media verifications, not every file considered); move row iteration queries from MediaRow to MediaDB.