PIG-3562: Implement combiner optimizations for DISTINCT (abain via cheolsoo)


git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1556673 13f79535-47bb-0310-9956-ffa450edef68
PIG-3647: Implement merge join in Tez

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1556961 13f79535-47bb-0310-9956-ffa450edef68
Initial commit