HADOOP-6218 exposed the internal "Location" object as a global Record
Sequence Number (RecNum). The feature is useful in a number of ways:
(1) support progress reporting for upper layers (object file, zebra);
(2) use RecNum as cursor by a secondary index; (3) support aligned
split across multiple parallel TFiles. Given that TFile is still at
its early stage of being adopted, I suggest that we port the patch
back to hadoop 0.20/0.21 now.