nameserver: make logging independent on internal variables

src/Makefile.am

src/knot/nameserver/axfr.c

src/knot/nameserver/axfr.h

src/knot/nameserver/internet.c

src/knot/nameserver/ixfr.c

src/knot/nameserver/log.h

src/knot/nameserver/notify.c

src/knot/nameserver/process_answer.c

src/knot/nameserver/process_answer.h

src/knot/nameserver/process_query.h

src/knot/nameserver/update.c