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

spec/requests/onebox_controller_spec.rb

@@ -56,7 +56,7 @@ describe OneboxController do
       stub_request(:get, url).to_return(status: 200, body: html).then.to_raise
 
       bypass_limiting
-      Rails.cache.delete("onebox__#{url}")
+      Discourse.cache.delete("onebox__#{url}")
       get "/onebox.json", params: { url: url }
       expect(response.status).to eq(200)
       expect(response.body).to include("Onebox1")