PIG-3602: Tear down TezSessions when Pig exits (rohini)

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1552777 13f79535-47bb-0310-9956-ffa450edef68
Initial troubleshooting for TestAccumulator
Add AccumulatorOptimizer
Update comment in TezPlanContainer