TAJO-584: Improve distributed merge sort.
TAJO-593: outer groupby and groupby in derived table causes only one shuffle output number.
TAJO-548: Investigate frequent young gc. (Min Zhou via hyunsik)
TAJO-592: HCatalogStore should supports RCFile and default hive field delimiter. (jaehwa)
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/incubator-tajo into TAJO-305