delayed jobs: reduce workers from 12 to 8.

Reduce worker count to reduce memory usage. Most are normally idle anyway.

config/deploy/production.rb

@@ -1,6 +1,6 @@
 set :user, "danbooru"
 set :rails_env, "production"
-set :delayed_job_workers, 12
+set :delayed_job_workers, 8
 append :linked_files, ".env.production"
 
 server "kagamihara", :roles => %w(web app cron), :primary => true