changeset: Init structures, do not malloc, better init.

src/knot/dnssec/zone-nsec.c

src/knot/dnssec/zone-sign.c

src/knot/nameserver/ixfr.c

src/knot/server/journal.c

src/knot/server/serialization.c

src/knot/updates/apply.c

src/knot/updates/changesets.c

src/knot/updates/changesets.h

tests/changeset.c