Cached costs

This is just for Nelson to review (not an actual pull request!).

Nelson, I've changed the balancing to be way faster now and it incorporates the changes I discussed with you today (bucketed times etc.). I did however have to change the cost algorithm to accommodate this. It's in CostBalancerStrategy, and it's called computeServerCostsFast. Can you take a look and let me know your thoughts?

Dhruv