TopologyAwareConsistentHashFactory doesn't distribute data to nodes evenly

When the topology of a cluster is "balanced" (i.e. all sites have the same number of racks, racks have the same number of machines, machines have the same number of nodes), the number of segments owned by each node should be approximately the same.

The current algorithm properly balances the primary owners of the segments, but the backup owners are not balanced, so a node can end up owning a lot more segments than expected.