Avoid re-authorizing if an existing authorization is still valid

[mithrandi]

Authorizations may last for much longer than certificates, we should check to see if a new authorization is in fact needed before authorizing.

[mithrandi]

Let's Encrypt have made some server-side changes in this area: https://community.letsencrypt.org/t/upcoming-change-valid-authz-reuse/16982/4

Need to investigate exactly how txacme interacts with these, I think we may be doing a bit more work than we need to, although everything should still work fine.

[mithrandi]

Since LE are reducing their authz validity period even further, this is of even less importance to us, so bumping the milestone.