[Reviewer: Rob] Add support for protocol independent (IPv4/IPv6) table indices based upon RFC 4001.

I believe this is a way to move forward from the IpAddress table indices used in the current MIB. I'm not sure it is the best way. It is more of an experiment, and a place to start the discussion of how to proceed.
I think this just does the wrong thing if addrStr is invalid (e.g. I think inet_pton won't allow IPv6 addresses with square brackets, which caught me out elsewhere recently). I'm not sure there's a lot we can do here (being inside SNMPd might restrict error logging), and I think the isValid() check catches it, but worth a comment at least?