Add sidekiq_job_attempt_threshold config variable

The same idea as Tadas Tamosauskas's commit, but implemented differently and for Sidekiq. I am happy to make whatever changes you'd like.
Hey @cdale77, sorry for the long delay on this. I think your statement is correct; the Proc block is run every time there is an exception, and you should be able to check the `sidekiq_job_attempt_threshold` configuration option there.