Balance tablets based on range hash buckets

When a user defines a schema that uses range + hash partitioning its is often the case that the tablets in the latest range, based on time or any semi-sequential data, are the only tablets that receive writes. Or even if not the latest, it is common for a single range to receive a burst of writes if backloading.

This is so common, that the default Kudu balancing scheme should consider placing/rebalancing the tablets for the hash buckets within each range on as many servers as possible in order to support the maximum write throughput. In that case, `min(#buckets, #total-cluster-tservers)` tservers will be used to handle the writes if the cluster is perfectly balanced. Today, even if perfectly balanced, it is possible for all the hash buckets to be on a single tserver.