Change Repartioner to swap among nodes from all specified zones.

PartitionBalance
- Added comment about possible weakness of our utility method.

Repartitoiner.java
- Changed random shuffle implementation to swap among all nodes in
  specified zones.
- Left TODOs and 'dead code' implemntation of intra-zone swapping vs
  inter-zone swapping.
Merge branch 'zone-expansion' of github.com:voldemort/voldemort into zone-expansion