events: reimplemented flush/xfer

src/knot/nameserver/axfr.c

src/knot/nameserver/axfr.h

src/knot/nameserver/internet.h

src/knot/nameserver/ixfr.c

src/knot/nameserver/ixfr.h

src/knot/nameserver/notify.c

src/knot/nameserver/notify.h

src/knot/nameserver/process_answer.c

src/knot/nameserver/process_query.c

src/knot/updates/xfr-in.c

src/knot/updates/xfr-in.h

src/knot/zone/events.c