What are you doing with ARI Retry-After?

[aarongable]

From the perspective of the ARI author, the ideal ACME client wakes up every hour, but only has to do local work most of the time: checking the latest ARI response to see if it's in the suggested window now, checking the latest retry-after to see if it needs to check again, checking the certs on disk to see if one has expired, checking the local config to see if there's a new cert to manage, etc. It only does "real" work if one of those checks says it needs to, so waking up that frequently is cheap.