Added parsing of IPv6 addresses for incoming mDNS requests

[fouge]

I want to have P2P communication fully based on IPv6, with local discovery using mDNS.
I am a beginner with libp2p so don't hesitate to bring your insights.

[lidel]

The idea SGTM, the new libp2p/specs/discovery/mdns (which afaik is not yet implemented in GO libs, but is in JS) lists both IPv4 and IPv6: https://github.com/libp2p/specs/blob/master/discovery/mdns.md#find-all-response

Someone familiar with the codebase needs to review this tho.

[Stebalien]

Thanks!

[Stebalien]

We can deduplicate some of this code.

var addr net.IP
if e.AddrV4 != nil {
    addr = e.AddrV4
} else if e.AddrV6 != nil {
    addr = e.AddrV6
} else {
    return some error
}

// construct the multiaddr

[Stebalien]

Thanks!