PERF: perform all cached counting in background (#15991)

Previously cached counting made redis calls in main thread and performed
the flush in main thread.

This could lead to pathological states in extreme heavy load.

This refactor reduces load and cleans up the interface

spec/rails_helper.rb

@@ -194,6 +194,8 @@ RSpec.configure do |config|
   config.infer_base_class_for_anonymous_controllers = true
 
   config.before(:suite) do
+    CachedCounting.disable
+
     begin
       ActiveRecord::Migration.check_pending!
     rescue ActiveRecord::PendingMigrationError