Fix #2619 - When redis feed is empty, fall back to database (#3721)

* Fix #2619 - When redis feed is empty, fall back to database

* Use redis value to return feed from database only while RegenerationWorker
hasn't finished running

* Fix specs

* Replace usage of reject!

app/services/precompute_feed_service.rb

@@ -17,6 +17,8 @@ class PrecomputeFeedService < BaseService
       statuses.each do |status|
         process_status(status)
       end
+
+      redis.del("account:#{@account.id}:regeneration")
     end
   end