This is a bottom up rewrite of Discourse cache to support faster performance
and a limited surface area.
ActiveSupport::Cache::Store accepts many options we do not use, this partial
implementation only picks the bits out that we do use and want to support.
Additionally params are named which avoids typos such as "expires_at" vs "expires_in"
This also moves a few spots in Discourse to use Discourse.cache over setex
Performance of setex and Discourse.cache.write is similar.
Discourse.cache is a more consistent method to use and offers clean fallback
if you are skipping redis
This is part of a larger change that both optimizes Discoruse.cache and omits
use of setex on $redis in favor of consistently using discourse cache
Bench does reveal that use of Rails.cache and Discourse.cache is 1.25x slower
than redis.setex / get so a re-implementation will follow prior to porting
