The authority now receives the ordinal in its constructor rather than a
global variable set at package initialization time. The ordinal is
passed via the command line option `--ordinal`.
Also, return early from ValidateChallenge if the challenge is already
valid. Interestingly, we aren't actually testing most of the
ValidateChallenge func, just the early error and return conditions. We
should add some more coverage here.
When constructing links we want to read the required host and protocol
information in a dynamic manner from the request for constructing ACME
links such as the directory information. This way, if the server is
running behind a proxy, and we don't know what the exposed URL should
be at runtime, we can construct the required information from the
host, tls and X-Forwarded-Proto fields in the HTTP request.
Inspired by the LetsEncrypt Boulder project (web/relative.go).
