Improve Hbase scan cost estimation.
Made changes to old fix based on code review comments.

Fix for bug 1333800
The real issue is MDAM plan is being costed high compared to single subset scan plan.
The reason is cost of HbaseScan node is estimated as if table has single region in mdam plan case
where as single subset scan plan costs for multi region table.

There already exists a logic to take into account number of regions for cost estimation. This code is
currently being used by single subset scan. I have moved this code to new function called ScanOptimizer::getNumActivePartitionsForHbaseTable() and call this function from both MDAM and single subset scan.

Change-Id: Ie1e7988d7ef5ffd745e92d3069eecdf05d5339c2
Merge "Improve Hbase scan cost estimation. Made changes to old fix based on code review comments."