ratelimits: Rename rate limits with identifiers

Now that many of our rate limits also cover IP address identifiers or ranges, their names have become confusing.

For the failed authorizations rate limits, "domain" has really meant "FQDN," and should now be renamed to "Identifier":

• FailedAuthorizationsPerDomainPerAccount
• FailedAuthorizationsForPausingPerDomainPerAccount

For CertificatesPerFQDNSet, the whole limit should be renamed to "DuplicateCertificates":

• CertificatesPerFQDNSet

We should also update our internal comments (and perhaps function names) about the "FQDNSet" database table, which is not easily renamed; its name has become deceptive.

Then, for the certificates per domain rate limits, "domain" has really meant "eTLD+1" all along - and now it includes "IPv4 address" and "IPv6 /64 range." We should choose a new term. Internally, we currently call this "covering identifier," which isn't bad but we should try to brainstorm something clearer:

• CertificatesPerDomain
• CertificatesPerDomainPerAccount

A change to the Let's Encrypt Web site's rate limits doc should accompany the Boulder change, which might also require updating anchor links in errors/errors.go.

Review the rate-limit naming references and errors/errors.go, then compare them with the rate-limits documentation at the linked Let's Encrypt website path. Decide consistent replacements for the domain, FQDN, and FQDNSet terms, update related comments or function names, and ensure documentation anchors and references match the Boulder changes.