IKEv2 host-to-host with NAT and CP.

ikev2-hostpair-01 reconnects using a different pre-NAT IP so it gets
a different port on the NAT router. This emulates switching network
without notify. It should cause east to REPLACE the connection due
to initial contact. It should also get the same lease ip as before.

On 3.23, this gives "cannot route -- route already in use" error,
and traffic flow stops