Time-based cache grouping

- [ ] Time-based grouper: In addition to MD5 structure, have
`/var/tmp/omnic/date-cache/2017/08/01/ae08/`. This will allow cron to
aggressively purge older cache, e.g. only maintain a couple days in prod.
This is good because IRL it won't be serving media anyway, the cache is only
good for conversion.

- [X] 'clearcache' command - delete a single foreign resource from cache,
removing all generated media from that foreign resource.
- [ ] Also support date-range (requires grouping)

Start by tracing the cache implementation and the existing clearcache command in the repository. Add time-based grouping alongside the MD5 structure, then support removing cached media by date range and verify that older date-grouped cache entries can be purged as described.

Written by the indexing model from the issue text.