IPv6 address scrambling should preserve special IPs and IP ranges

[dhalperi]

The anonymizer should preserve known IPs and/or prefixes as appropriate. The stanardized list of known prefixes is here:

https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml

Decisions may need to be made on a case-by-case basis.

(To implement this, we'll probably want to generalize the should_anonymize functionality to handle partial preservations.)