STORM-132 sort supervisor by free slot in desending order

Sort supervisor by free slot in desending order in EvenScheduler to schedule more evenly between supervisor
Could we do something like this?
```
(sort #(> (count %1) (count %2)) (vals (group-by first all-slots)))
```
It would be nice to move this into its own function, so we can write a quick unit test for it.