Celluloid requires at least 2 workers
Raise arg error if cache file path dir is not writable
Only use FileWarmer on initialize if the cache file is readable
Raise if warmer doesn't include Warmer module, or doesn't have a public instance method for perform
Fix method calls where name was changed
Create rake tasks firebolt:cache:{purge,warm,rebuild}

- The `purge` task deletes all known keys with the current salt.
- The `warm` task simply warms the cache with the configured warmer
(not the FileWarmer).
- The `rebuild` task performs a purge and then warm.