zone: refactoring: zone_t points to server_t

src/knot/ctl/commands.c

src/knot/dnssec/zone-events.c

src/knot/events/handlers/ds_check.c

src/knot/events/handlers/nsec3resalt.c

src/knot/updates/zone-update.c

src/knot/zone/backup.c

src/knot/zone/zone.c

src/knot/zone/zone.h

src/knot/zone/zonedb-load.c

src/utils/kzonesign/main.c

tests/knot/test_server.h

tests/knot/test_zone-update.c