DEV: Implement a faster Discourse.cache

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.

app/controllers/email_controller.rb

@@ -112,7 +112,7 @@ class EmailController < ApplicationController
     else
 
       key = "unsub_#{SecureRandom.hex}"
-      $redis.setex key, 1.hour, user.email
+      Discourse.cache.write key, user.email, expires_in: 1.hour
 
       url = path("/email/unsubscribed?key=#{key}")
       if topic
@@ -125,7 +125,7 @@ class EmailController < ApplicationController
   end
 
   def unsubscribed
-    @email = $redis.get(params[:key])
+    @email = Discourse.cache.read(params[:key])
     @topic_id = params[:topic_id]
     user = User.find_by_email(@email)
     raise Discourse::NotFound unless user