child-delete: Don't send delete for expired CHILD_SAs that were already rekeyed

child-delete: Don't send delete for expired CHILD_SAs that were already rekeyed

The peer might not have seen the CREATE_CHILD_SA response yet, receiving a
DELETE for the SA could then trigger it to abort the rekeying, causing
the deletion of the newly established SA (it can't know whether the
DELETE was sent due to an expire or because the user manually deleted
it).  We just treat this SA as if we received a DELETE for it.  This is
not an ideal situation anyway, as it causes some traffic to get dropped,
so it should usually be avoided by setting appropriate soft and hard limits.

References #2815.