Prior to this PR the SA's `CountRegistrationsByIP` treated IPv6
differently than IPv4 by counting registrations within a /48 for IPv6 as
opposed to exact matches for IPv4. This PR updates
`CountRegistrationsByIP` to treat IPv4 and IPv6 the
same, always matching exactly. The existing RegistrationsPerIP rate
limit policy will be applied against this exact matching count.
A new `CountRegistrationsByIPRange` function is added to the SA that
performs the historic matching process, e.g. for IPv4 it counts exactly
the same as `CountRegistrationsByIP`, but for IPv6 it counts within
a /48.
A new `RegistrationsPerIPRange` rate limit policy is added to allow
configuring the threshold/window for the fuzzy /48 matching registration
limit. Stats for the "Exceeded" and "Pass" events for this rate limit are
separated into a separate `RegistrationsByIPRange` stats scope under
the `RateLimit` scope to allow us to track it separate from the exact
registrations per IP rate limit.
Resolves https://github.com/letsencrypt/boulder/issues/2738
fbd87b1757