Improve CRON execution.

[sfmskywalker]

"get the next occurence, reinit your reference to this occurence and get again the next occurence, so that you can compute the time span between 2 occurences. Then you will have to wait from now to now + this time span.

Maybe we could have an option to use the cron expression as a time interval.

If unchecked : The timer will trigger on the next occurence defined by the cron expression and following the time the timer has been started. Note: the timer could trigger immediately.

If checked : use the cron expression as a time interval, so that when started the timer will trigger after this time interval. The time interval being the time between 2 occurences as defined by the cron expression.

Note: knowing that right now the granularity of bg tasks is 1 minute."

See #1378 (comment)