Rename Authenticator.server_{cert,key}_fingerprint

[reynir]

By dropping the server_ prefix. The functions do not check that the certificate have extended key usage of server auth and could just as well be used to authenticate clients (although the server case is more likely). The old names are kept as aliases and a deprecation attribute is added.

This may result in some churn, but judging from sherlocode it doesn't look too bad.