HIVE-4843: Refactoring MapRedTask and ExecDriver for better re-usability and readability (Vikram Dixit K via Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1509081 13f79535-47bb-0310-9956-ffa450edef68
HIVE-4827: Merge a Map-only task to its child task (Yin Huai via Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1509082 13f79535-47bb-0310-9956-ffa450edef68