Put running downtimes in effect

Put running downtimes in effect

If Icinga2 was restarted with a newly configured downtime that should
be in effect at the time of restart, the should-be-running segment of
it was not put into effect.

Add new LegacyTimePeriod::FindRunningSegment() and
ScheduledDowntime::FindRunningSegment() functions, call the latter in
ScheduledDowntime::CreateNextDowntime() before trying the old
ScheduledDowntime::FindNextSegment().