FIX: Don't use DistributedCache to store redis readonly state

This can cause unbound CPU usage in some cases, and excessive logging in other cases. This commit moves redis readonly information into the local process, but maintains the DistributedCache for postgres readonly state.

app/controllers/application_controller.rb

@@ -139,7 +139,7 @@ class ApplicationController < ActionController::Base
   end
 
   rescue_from PG::ReadOnlySqlTransaction do |e|
-    Discourse.received_readonly!
+    Discourse.received_postgres_readonly!
     Rails.logger.error("#{e.class} #{e.message}: #{e.backtrace.join("\n")}")
     raise Discourse::ReadOnly
   end