new_node: Zone and RRSet API cleanup

src/knot/updates/ddns.c

src/knot/updates/xfr-in.c

src/knot/zone/node.c

src/knot/zone/zone-contents.c

src/knot/zone/zone-contents.h

src/libknot/packet/pkt.c

src/libknot/rrset.c

src/libknot/rrset.h