HIVE-5770: Switch merge tasks to use tez (Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/tez@1539560 13f79535-47bb-0310-9956-ffa450edef68
HIVE-5738: Need to clear global work in ExecMapper/ExecReducer for container reuse in tez (Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/tez@1539573 13f79535-47bb-0310-9956-ffa450edef68
HIVE-5772: Print message for union operators (Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/tez@1539585 13f79535-47bb-0310-9956-ffa450edef68