[5.0] [trunk] ir.cron and its implementation use a busywait timer that consumes unnecessary resources

ir.cron is a netsvc.Agent and the setAlarm() method used to schedule tasks is implemented using python's threading.Timer which ultimately relies on a threading.Condition.
The issue is that threading.Condition's wait method is implemented with a busywait algorithm that consumes small amounts of system resources every second even if the task is scheduled to execute several days later.

This becomes a problem when a lot of Agent tasks (e.g. ir.cron tasks) are scheduled, because that creates as many threading.Condition as there are tasks, and the system resources can become impacted by the sum of all the little amounts of memory and CPU consumed.

This could become an important issue on large multi-tenant deployments...

An implementation based on real sleep() calls should be considered to avoid this.