new_node: Bugfixes, storing old and new data in zone_t.

src/knot/nameserver/update.c

src/knot/server/zones.c

src/knot/updates/ddns.c

src/knot/updates/xfr-in.c

src/knot/updates/xfr-in.h

src/knot/zone/zone-contents.h

src/knot/zone/zone.c

src/knot/zone/zone.h