Batch / aggregate Load Balancer adds in convergence [AS-398]

This adds an optimization function for taking a bag of steps and optimizing them, and implements an optimizer for merging multiple calls to add LB nodes into one per LB.

This is based on PR #719 so the diff should be smaller than it looks. Diff of just this branch against its upstream: https://github.com/rackerlabs/otter/compare/always-use-configured-lb...batch-lb-adds