Allow nodes to learn their own reflexive UDP address.

When a node gets a succesful UDP probe reply, it informs the peer of the
UDP address and port that it has. The peer can then use this information to
inform other nodes it wants to communicate with.

Currently, this is only done to close the time window where two nodes have
established an SPTPS session via a third node, while the third node hasn't
learned of the two nodes' UDP addresses, in which case it was too late for
the third node to assist with hole punching.