process_answer: stub implementation

src/knot/nameserver/axfr.c

src/knot/nameserver/ixfr.c

src/knot/nameserver/notify.c

src/knot/nameserver/process_query.c

src/knot/nameserver/process_query.h

src/knot/nameserver/update.c

src/knot/server/tcp-handler.c

src/knot/server/udp-handler.c

tests/process_query.c