Fix rubocop issues, introduce usage of frozen literal to improve performance

app/services/precompute_feed_service.rb

@@ -1,10 +1,10 @@
+# frozen_string_literal: true
+
 class PrecomputeFeedService < BaseService
   # Fill up a user's home/mentions feed from DB and return a subset
   # @param [Symbol] type :home or :mentions
   # @param [Account] account
   def call(type, account)
-    instant_return = []
-
     Status.send("as_#{type}_timeline", account).limit(FeedManager::MAX_ITEMS).each do |status|
       next if FeedManager.instance.filter?(type, status, account)
       redis.zadd(FeedManager.instance.key(type, account.id), status.id, status.reblog? ? status.reblog_of_id : status.id)
@@ -14,6 +14,6 @@ class PrecomputeFeedService < BaseService
   private
 
   def redis
-    $redis
+    Redis.current
   end
 end