[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.)
This is an indication that you should do something different! You're recomputing the largest container with every comparison, which seems bad. Compute once and cache. When you do that, you can just exclude machines that return an error.