Tweaks to PartitionBalance and RebalancePlan

PartitionBalance
- calculate partition-stores per zone.
- This measure provides more context to evaluate the size of any plans to rebalance the cluster.

RebalancePlanCLI
- fix typo in verbose usage message
Initial hack at new rebalance scheduler.

Added RebalanceController.scheduler
- limits each node to participating in a single task as either a stealer or donor.
- randomizes the order in which tasks are attempted to be scheduled
- not a clean implementation, but enough to evaluate.