HIVE-4002 Fetch task aggregation for simple group by query (Navis Ryu and Yin Huai via egc)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1519306 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-YAHOO-321-trunk-gd