Depending on the lifetimes a CHILD_SA we rekeyed as responder might
expire shortly afterwards. We don't want to rekey it again.
When retrying due to an INVALID_KE_PAYLOAD notify the expected state
is CHILD_REKEYING if it is anything else (e.g. due to a collision) we
ignore it.
We also abort the exchange properly if we don't find the CHILD_SA, no
need for an empty INFORMATIONAL exchange anymore.