rrset: add knot_rrset_deep_copy_no_sig()

- needed for zone diff and DNSSEC
- knot_rrset_deep_free_no_sig() already exists
DNSSEC: implement knot_zone_sign_update_soa()

Update changeset by filling old SOA and new SOA with incremented serial.
remote signzone: temporary code to apply the changeset

- changes are written into journal
- from some reason the zone struct is not set correctly