JIRA: https://issues.apache.org/jira/browse/SPARK-9173

Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #7540 from yjshen/union_pushdown and squashes the following commits:

278510a [Yijie Shen] rename UnionPushDown to SetOperationPushDown
91741c1 [Yijie Shen] Add UnionPushDown support for intersect and except