WIP: zone_update: fixed serial_from in update's changeset
WIP: @dsalzman think a little if the fix is optimal!
this bug bahaves like this: if you configure: dnssec-signing: on zonefile-load: difference it will, upon changing zonefile, make inconsistencies in journal leading in broken IXFR
the reason is that by calling set_new_soa over existing SOA in changeset (probably because of zone_update_from_differences) it updates the changeset's soa_from to incorrect value