ike-rekey: Handle undetected collisions also if delete is delayed

ike-rekey: Handle undetected collisions also if delete is delayed

If the peer does not detect the rekey collision and deletes the old
IKE_SA and then receives the colliding rekey request it will respond with
TEMPORARY_FAILURE.  That notify may arrive before the DELETE does, in
which case we may just conclude the rekeying initiated by the peer.

Also, since the IKE_SA is destroyed in any case when we receive a delete
there is no point in storing the delete task in collide() as process_i()
in the ike-rekey task will never be called.