[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.
https://github.com/Metaswitch/clearwater-docs/wiki/Clearwater-SNMP-Statistics for Project Clearwater, so uses of older versions know which MIB file to use.