Future development:

- documentation/cookbook for moving from Cache::Cache to Cache::CacheFactory.
- Cache::CacheFactory::Expiry::Base->set_purge_order().
- update_last_accessed_at option.
- weaken_refs option to combine with no_deep_clone?
- Other storage policies:
  - Cache::BerkeleyDB
    - requires BerkeleyDB.
- Expiry policies:
  - 'all' pruning policy - just clears entire cache on purge().
  - 'entries' pruning policy - prune if there's more than X keys.
- auto_purge options:
  - Allow auto_purge on X set/get operations as well/instead of interval.
- get/load callback functions?
- refactor to complain about unhandled options to stop typo problems.
- Add "performance" section to documentation.
- optimize calls to foreach_*() functions:
  - pass named function refs.
  - don't call if no policies of that type.
  - pre-count policy numbers to speed up check.