Cache cleanup review

Cache cleanup review

Fixes several XXXs.

- Separate node->mtim into attempt_ts and success_ts.
  Because they're really two different timestamps; The former is meant
  for node expiration, the latter for HTTP IMS.
- Move removal of orphaned fallbacks to remove_abandoned().
  Because orphaned refreshes need the same logic.
- Added the (randomly missing) expiration threshold for orphans.

It's still missing the implementation of remove_orphaned_files(),
but I'm still weighting options, as it seems it's going to be an
expensive operation that's rarely going to do anything.