Memoize (& lazily load) rack information for each broker
Memoize (& lazily load) rack information for each broker

When I attempted to rebalance a cluster with > 1000 topics it took quite
a lot of time to even generate the plan, as for each topic it'll do a
bunch of requests to ZooKeeper. The more the merrier isn't always the
case.
burtcorp/ktl