Future development:

- documentation/cookbook for moving from Cache::Cache to Cache::CacheFactory.
- Cache::CacheFactory::Expiry::Base->set_purge_order().
- update_last_accessed_at option.
- no_deep_copy option for setting, turns off deep clone of data on set().
  - weaken_refs option to combine with this?
- Other storage policies:
  - Cache::BerkeleyDB
  - Cache::FastMemoryCache (turns on no_deep_copy automagically?)
- 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.
- update lastmodified tests to ensure file is created and touched.
