When creating a fixed downtime that starts immediately while the checkable is
in a non-OK state, previously the code path for flexible downtimes was used to
trigger this downtime. This is fixed by this commit which resolves two issued:
1. Missing downtime start notification: notifications work differently for
fixed and flexible downtimes. This resulted in missing downtime start
notifications under the conditions described above.
2. Incorrect downtime trigger time: this code path would incorrectly assume the
timestamp of the last checkable as the trigger time which is incorrect for
fixed downtimes.