DEV: use Discourse.cache over Rails.cache

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

lib/discourse.rb

@@ -591,6 +591,7 @@ module Discourse
     SiteSetting.after_fork
     $redis._client.reconnect
     Rails.cache.reconnect
+    Discourse.cache.reconnect
     Logster.store.redis.reconnect
     # shuts down all connections in the pool
     Sidekiq.redis_pool.shutdown { |c| nil }