Make sharding balancer smarter when asymmetric hardware

Make the balancer shard size aware, and design it to use all available storage.

e.g. If a shard set currently consisting of shards with 2TB capacity is extended by adding some 3TB shards, only 2TB of the larger shards will currently be used.

Given that sharding is mongo's "scale out" strategy, this would be a major improvement in allowing projects to scale out much more economically.