Determine peer's reflexive address and port when exchanging keys.

To help peers that are behind NAT connect to each other directly via UDP, they
need to know the exact external address and port that they use. Keys exchanged
between NATted peers necessarily go via a third node, which knows this address
and port, and can append this information to the keys, which is in turned used
by the peers.

Since PMTU discovery will immediately trigger UDP communication from both sides
to each other, this should allow direct communication between peers behind
full, address-restricted and port-restricted cone NAT.