zone: cleanup of zone, renaming, documentation

src/knot/ctl/remote.c

src/knot/dnssec/nsec-chain.c

src/knot/dnssec/nsec-chain.h

src/knot/dnssec/nsec3-chain.c

src/knot/dnssec/zone-nsec.c

src/knot/dnssec/zone-sign.c

src/knot/nameserver/axfr.c

src/knot/server/zone-load.c

src/knot/updates/apply.c

src/knot/updates/apply.h

src/knot/zone/contents.c

src/knot/zone/contents.h

src/knot/zone/events.c

src/knot/zone/zone-diff.c

src/knot/zone/zone-diff.h

src/knot/zone/zone-load.c

src/knot/zone/zone-tree.c

src/knot/zone/zone-tree.h

src/knot/zone/zone.h

src/knot/zone/zonedb-load.h