Replaced direct use of node's fields with API

src/dnslib/response.c

src/dnslib/response2.c

src/dnslib/zone-contents.c

src/dnslib/zone-dump.c

src/dnslib/zone-load.c

src/dnslib/zonedb.c

src/knot/server/name-server.c

src/knot/server/xfr-in.c

src/zcompile/zcompile.c