PIG-3846: Implement automatic reducer parallelism

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1603243 13f79535-47bb-0310-9956-ffa450edef68
PIG-3983: TestGrunt.testKeepGoigFailed fail on tez mode

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1603245 13f79535-47bb-0310-9956-ffa450edef68