Avoids collisions of Downtime.id between schedulers

Each scheduler starts Downtime.id from 1 and increments it by 1 at each
downtime creation. So, when running multiple schedulers, multiple
downtimes will have the same ID. Commands like DEL_SVC_DOWNTIME are
broadcasted to all schedulers for execution. When deleting a downtime,
if the ID is not unique, all the downtimes that have the same ID will be
deleted. By using microseconds since 1970-01-01 as an ID, the probability
of collision is reduced.

We used this patch for the last few days, it works fine for us. It has been tested with Thruk and our internal software that manages downtimes.