PIG-3493: Add max/min for datetime

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.12@1528724 13f79535-47bb-0310-9956-ffa450edef68
PIG-3493: Add max/min for datetime

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.12@1528725 13f79535-47bb-0310-9956-ffa450edef68