x/crypto/acme/autocert: Manager.GetCertificate should check host policy before consulting cache

[rolandshoemaker]

Manager.GetCertificate doesn't check the host policy if the request is a challenge, probing the cache if there isn't an entry in the certTokens map. While cache probes should be cheap, an attacker may exploit any latency introduced by these probes by forcing repeated cache misses.

This can be fixed by simply moving the host policy check above the cache probe, which is likely to be cheaper, if implemented, than a cache probe.

This is a PUBLIC track security issue, due to its minimal impact.

Thanks to Dimitri Stiliadis for reporting this issue.