[SPARK-9241][SQL] Supporting multiple DISTINCT columns (2) - Rewriting Rule

The second PR for SPARK-9241, this adds support for multiple distinct columns to the new aggregation code path.

This PR solves the multiple DISTINCT column problem by rewriting these Aggregates into an Expand-Aggregate-Aggregate combination. See the [JIRA ticket](https://issues.apache.org/jira/browse/SPARK-9241) for some information on this. The advantages over the - competing - [first PR](https://github.com/apache/spark/pull/9280) are:
- This can use the faster TungstenAggregate code path.
- It is impossible to OOM due to an ```OpenHashSet``` allocating to much memory. However, this will multiply the number of input rows by the number of distinct clauses (plus one), and puts a lot more memory pressure on the aggregation code path itself.

The location of this Rule is a bit funny, and should probably change when the old aggregation path is changed.

cc yhuai - Could you also tell me where to add tests for this?

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #9406 from hvanhovell/SPARK-9241-rewriter.