Add GoodJob.current_thread_running? and GoodJob.current_thread_shutting_down? for graceful shutdowns

[bensheldon]

When called within the context of a performing job, these methods will return the status of thread's scheduler and indicate when it has started a graceful shutdown (this is necessary because GoodJob can contain multiple schedulers that can be independently operated).

Simplest implementation. Looking for feedback on:

• Naming
• How this should report when running inline
• Is a global desired? or would it be better as a job extension with a local good_job_shutting_down? method.

Connects to #1217 1217