Let SIP handle multiple IP Addresses per client

This pull requests enables:

* Listen to IPv6 Connections
* Send all possible IP Addresses to peers, clients should try each of those until they can connect through one
* Peers that are only reachable via IPv6 may now be reached by other IPv6 enabled peers
* Peers that are behind the same NAT and not in the same broadcast domain can now connect
* Connecting works even if one client is assumed as visible but a firewall blocks port 50210 (both need to be visible and only of one them could be blocked)