timers: update timers just when server shutdown or reload

src/knot/main.c

src/knot/nameserver/notify.c

src/knot/zone/events/events.c

src/knot/zone/events/events.h

src/knot/zone/events/handlers.c

src/knot/zone/timers.c

src/knot/zone/timers.h

src/knot/zone/zonedb-load.c

tests/zone_timers.c