Improve PostStatusService performance (#7317)

Offload creation of local notifications to a worker. Remove two
redundant SQL queries from ProcessMentionsService, remove n+1
XML/JSON serialization via memoization

app/workers/local_notification_worker.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class LocalNotificationWorker
+  include Sidekiq::Worker
+
+  def perform(mention_id)
+    mention = Mention.find(mention_id)
+    NotifyService.new.call(mention.account, mention)
+  rescue ActiveRecord::RecordNotFound
+    true
+  end
+end