HIVE-7567 - support automatic calculating reduce task number (Chengxiang Li via Brock) [Spark Branch]

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/spark@1616085 13f79535-47bb-0310-9956-ffa450edef68
HIVE-7492: Enhance output collector for Hive(Map|Reduce)Function.