cmd/icingadb-migrate: ignore icinga_downtimehistory#was_started=0 rows

Such appear e.g. for scheduled but never triggered flexible Downtimes.
Not triggered = no events.

cmd/icingadb-migrate/convert.go

@@ -220,6 +220,7 @@ type downtimeRow = struct {
 	Duration            int64
 	ScheduledStartTime  int64
 	ScheduledEndTime    int64
+	WasStarted          uint8
 	ActualStartTime     int64
 	ActualStartTimeUsec uint32
 	ActualEndTime       int64
@@ -242,6 +243,10 @@ func convertDowntimeRows(
 	for _, row := range idoRows {
 		checkpoint = row.DowntimehistoryId
 
+		if row.WasStarted == 0 {
+			continue
+		}
+
 		id := calcObjectId(env, row.Name)
 		typ := objectTypes[row.ObjecttypeId]
 		hostId := calcObjectId(env, row.Name1)