Use thread queue delivery method for Resque, when it's safe to do so

[imjoehaines]

Goal

Our thread queue delivery relies on an at_exit hook to flush the queue before the thread exits. Unfortunately this doesn't work with Resque as it does not run on_exit hooks by default. There is a run_at_exit_hooks setting, which can be used to enable on_exit hooks and so we can safely use thread queue delivery when this is enabled

Related to #593

Changeset

• Detect if run_at_exit_hooks is enabled and, if so, use thread queue delivery (otherwise stick with synchronous)
• Updated the Resque Maze Runner test to enable run_at_exit_hooks

Testing

I've manually tested that delivery succeeds with thread queue delivery and on_exit hooks (and that it fails with on_exit hooks disabled) and there's a new Maze Runner test to ensure this doesn't regress