zone_update: apply changes right away

src/knot/dnssec/nsec-chain.c

src/knot/dnssec/zone-nsec.c

src/knot/dnssec/zone-sign.c

src/knot/nameserver/ixfr.c

src/knot/server/journal.c

src/knot/updates/apply.c

src/knot/updates/apply.h

src/knot/updates/changesets.c

src/knot/updates/changesets.h

src/knot/updates/ddns.c

src/knot/updates/zone-update.c

src/knot/updates/zone-update.h

src/knot/zone/zone-diff.c

tests/changeset.c

tests/journal.c

tests/zone_update.c