PIG-3835: Improve performance of union (rohini)

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1583768 13f79535-47bb-0310-9956-ffa450edef68
PIG-3803: POPoissonSample should handle endOfAllInput flag

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1583799 13f79535-47bb-0310-9956-ffa450edef68
PIG-3829: Make custom counter work

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1583817 13f79535-47bb-0310-9956-ffa450edef68