IPv6 Candidate pairing

[palerikm]

Need to update how those a paired.

Make sure:

• Only link local to link local (No link local to other..)
• no loopback addresses (Should library check or up to client?)
• No Relay to any private address (IPv4 and IPv6)

[oeichenwei]

I think it should be better be handled in application layer, we even need to validate the preferred expire time for temporary address.