PIG-3812: Several clean-ups in PigStats and JobStats (cheolsoo)


git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1578679 13f79535-47bb-0310-9956-ffa450edef68
PIG-3816: Incorrect Javadoc for launchPlan() method

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1578746 13f79535-47bb-0310-9956-ffa450edef68