zone API: initial commit of zone+changeset hybrid

Knot.files

src/Makefile.am

src/knot/dnssec/nsec3-chain.c

src/knot/updates/apply.c

src/knot/updates/ddns.c

src/knot/updates/zone-update.c

src/knot/updates/zone-update.h

src/knot/zone/contents.c

src/knot/zone/node.c

src/knot/zone/node.h

src/knot/zone/zone-tree.c

tests/Makefile.am

tests/node.c

tests/zone-update.c