autonat: provide different reachability states and callbacks for IPv6 and IPv4

[Jorropo]

The default IPv6 setup on buisness and home ISP is to have a statefull NAT preventing non solicited inbound IPv6 packets.
However a good share of them also support upnp but only for ipv4.

This lead to missleading autonat results that tell us:

I'm reachable !!

Even tho you are only really reachable on IPv4. This is an issue because I'm trying to get Kubo working on IPv6 networks, because the sad reality is that right now things are quite broken if you do not have an ipv4 uplink.
Likely we are gonna have to split the current DHT in an IPv4 and IPv6 section.

[Jorropo]

For the implementation it does not sounds that bad, we would duplicate the statemachine and each statemachine would only gives our observed IPv4 or IPv6 as well as establish matching connections to other autonat peers.

[marten-seemann]

This sounds like it would be solved by the new address pipeline (and AutoNAT v2, which @sukunrt is working on right now). This will allow us to determine the reachability status for each address individually, which would reveal the different treatment of IPv4 and IPv6 you describe.