PIG-4117: Implement merge cogroup in Tez

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1617665 13f79535-47bb-0310-9956-ffa450edef68
PIG-4117: Implement merge cogroup in Tez

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1617666 13f79535-47bb-0310-9956-ffa450edef68