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
PIG-3367: Add assert keyword (operator) in pig (aniket486)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1525045 13f79535-47bb-0310-9956-ffa450edef68
PIG-3410: LimitOptimizer is applied before PartitionFilterOptimizer - missing files (aniket486)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1525096 13f79535-47bb-0310-9956-ffa450edef68
PIG-3471