PIG-3457: Provide backward compatibility for PigStatsUtil and JobStats

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1524532 13f79535-47bb-0310-9956-ffa450edef68
Merged latest trunk changes to tez branch


git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1525017 13f79535-47bb-0310-9956-ffa450edef68