CASSANDRA-6091 – Better Vnode support in hadoop/pig

 After all splits have been fetched (or "described")
  group them by "dataNodes", sort them by startToken,
  and join them together if possible (and still within split size).

 There remains a bug here, see the assert statement commented out on line 253.
  Sorting the splits from describe_local_ring seems to show duplicate token ranges.