Merge branch 'xfr_failovers' into 'master'

src/knot/events/handlers/refresh.c

src/knot/journal/serialization.c

src/knot/updates/zone-update.c

src/knot/zone/contents.c

src/knot/zone/contents.h

src/knot/zone/zone.c

src/knot/zone/zone.h

tests-extra/tests/axfr/failover/test.py

tests-extra/tools/dnstest/server.py

tests-extra/tools/dnstest/zonefile.py