Add cron_graceful_restart_period to avoid missing recurring jobs that occurred during deployment downtime

[bensheldon]

Closes #1484

[Earlopain]

If I use this in conjunction with preserve_job_records = false then this will double-enqueue, right? I'm not sure how this behaves even right now, I guess it probably works (as long as process clocks are close enough)?

[bensheldon]

Good question! Cron'ed jobs will ignore the preserve_job_records configuration and always be preserved (until they're automatically cleaned up): #767

[Earlopain]

Ah right, good to know 👍. I was looking at that piece of code by searching for preserve_job_records but missed the relevant condition to the right

[tjefferson08]

very cool, thanks for this feature 👍 all your heroku users thank you!