Implement IPv6 DNS records creation

[KuxaBeast]

Solves #48 and #50.

Implementation details

IPv6 records are created based on DHCPv4 leases, so this feature is only useful on dual-stack deployments. The AAAA records are only created if neighbor discovery returns reachable status and removed only if not reachable or stale.

The separation of networks is done by the DHCP servers' bound interfaces rather than by the network domains. This implementation made more sense, imho, even though there might be similar records across multiple DHCP networks with the same domain set.