knot: unify DS-related event terminology

Knot.files

src/knot/Makefile.inc

src/knot/dnssec/key-events.c

src/knot/dnssec/zone-events.h

src/knot/events/events.c

src/knot/events/events.h

src/knot/events/handlers.h

src/knot/events/handlers/dnssec.c

src/knot/events/handlers/ds_check.c

src/knot/events/replan.c

src/knot/modules/onlinesign/onlinesign.c

src/knot/zone/timers.c

src/knot/zone/timers.h

tests/knot/test_zone_timers.c