changesets: Removed changesets structure.

src/knot/dnssec/zone-events.c

src/knot/nameserver/ixfr.c

src/knot/nameserver/update.c

src/knot/server/journal.c

src/knot/server/journal.h

src/knot/server/serialization.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/zone/events.c

src/knot/zone/zone-load.c

src/knot/zone/zone.c

src/knot/zone/zone.h