rate-limit retry in case of previous error

When the challenge fails for some reason, auto-ssl will retry upon each request: this is bad because the letsencrypt account will quickly become rate-limited after five failed attempts.

So i suggest auto-ssl sets a rate limit of 4 failures an hour for a given domain.

Start by locating the Lua request and challenge-retry path, then inspect how state can be tracked per domain. Define how failures are counted within an hour and verify the behavior against the Let's Encrypt rate limit; done means a domain stops retrying after four failures in an hour without affecting other domains.

Written by the indexing model from the issue text.