LoopingCall sleep interval does not account for duration of last tasks run

[Eoghan Glynn]

The time spent executing tasks is not accounted for when determining how much time to sleep between task runs.

So for example if periodic_interval is set to N seconds, the actual time between tasks is of the order of:

       N + \Sigma duration(task_i)/(1 + ticks for task_i)

The more tasks with ticks=0, and the longer the task duration, the more we skew away from tasks executing on wall-clock boundaries.

As discussed on:

  http://lists.openstack.org/pipermail/openstack-dev/2012-November/002921.html