The latest versions of draft-ietf-acme-tls-alpn specify a different
idPeAcmeIdentifierV1 than LEGO was previously using. The old value had
a conflict with an existing assignment.
This commit updates the idPeAcmeIdentifierV1 value to match draft-05 and
updates any references to the draft RFC to use the latest draft number.
* refactor: create log.Infof and log.Warnf
* refactor: review DNS providers.
- use one `http.Client` by provider instead of one client by request
- use the same receiver name `d` for all `DNSProvider`
- use `http.MethodXXX`
* refactor: logger init.
