[WIP] Least load scheduler: a simple version

This PR is trying to implement a least load scheduler as mentioned in #471. It is still under development but feel free to make any comment. (This PR depends on #448. I will clean the commit history later.)
I'd rather us add up the resources in the pods assigned to the machine. What you've written will overcommit. That's an optimization that we need users to explicitly ask for, IMO, because it means they're not really getting all the resources they want. For the first pass, I don't think we should be this fancy.