Fix mysql deadlocks while reserving jobs

[ljfranklin]

• We frequently see deadlock errors when using mysql and a large number
  of workers
• This PR adds a fix for the deadlocks as well as modifying a test to
  exposes the deadlock issue in the previous implementation
• This implementation is based on the reserve implementation from the
  default active_record backend:
  https://github.com/collectiveidea/delayed_job_active_record/blob/42a68ed79917e14244a25a0a77fef0924bef3e97/lib/delayed/backend/active_record.rb#L64
• You can see the original error by pulling in the new test and running:
  DEBUG=true DB=mysql bundle exec rspec spec/delayed/backend/sequel_spec.rb:27

[ljfranklin]

@JonathanTron gentle bump on this. Not sure if this repo is still maintained, if not we can keep our fork going. Thanks!

[JonathanTron]

Hi @ljfranklin, thanks for reporting the issue and providing a PR.

We're not using MySQL at all, I'm willing to accept the PR, but could you do it like in the ActiveRecord version and only apply this fix if using MySQL?

[ljfranklin]

@JonathanTron sure, pushed that update. And if you're curious, here's an overly long thread about people arguing about deadlocks in the active_record backend: collectiveidea/delayed_job_active_record#63.

[JonathanTron]

@ljfranklin Thanks a lot for the fast action!