Cooldown scale to instances and priority improvements

- Cooldown now scales to # of instances
- Cooldown ignores tasks which were cleaned by Singularity
- Tasks which are all due are now ordered by the last timestamp when a task for that request was started. This means tasks which have not ran recently will take priority (on a single offer basis) over tasks which have ran recently. This should help protect against cases where a single task is monopolizing resources.