Keep scheduler jobs unique until they're done (#8287)

app/workers/scheduler/backup_cleanup_scheduler.rb

app/workers/scheduler/doorkeeper_cleanup_scheduler.rb

app/workers/scheduler/email_scheduler.rb

app/workers/scheduler/feed_cleanup_scheduler.rb

app/workers/scheduler/ip_cleanup_scheduler.rb

app/workers/scheduler/media_cleanup_scheduler.rb

app/workers/scheduler/subscriptions_cleanup_scheduler.rb

app/workers/scheduler/subscriptions_scheduler.rb

app/workers/scheduler/user_cleanup_scheduler.rb