Instead of rekeying, delete a quick mode if we have a fresher instance

Instead of rekeying, delete a quick mode if we have a fresher instance

If both peers initiate quick mode rekeying simultaneously, we end up
with duplicate SAs for a configuration. This can't be avoided, nor do
the standards provide an appropriate solution. Instead of closing one
SA immediately, we keep both. But once rekeying triggers, we don't
refresh the SA with the shorter soft lifetime, but delete it.