zone-size-limit: axfr, ixfr transfer size limit

src/knot/conf/scheme.c

src/knot/conf/scheme.h

src/knot/nameserver/axfr.c

src/knot/nameserver/axfr.h

src/knot/nameserver/ixfr.c

src/knot/zone/contents.h

src/knot/zone/semantic-check.c

src/libknot/rrset.c

src/libknot/rrset.h

tests-extra/tests/ixfr/transfer_size/data/example.com.zone

tests-extra/tests/ixfr/transfer_size/data/example.com.zone.1

tests-extra/tests/ixfr/transfer_size/data/example.com.zone.2

tests-extra/tests/ixfr/transfer_size/test.py

tests-extra/tools/dnstest/server.py