Pushdown DISTINCT agg fields as (additional)  GROUP BY keys

DISTINCT aggs like COUNT DISTINCT are expensive. But in most cases it is helpful to push down these as group by keys before counting them.

SELECT agg(x), COUNT(DISTINCT y) FROM T GROUP BY z;

can be rewritten as 

SELECT agg(x), COUNT(DISTINCT y) FROM (SELECT partial_agg(x) AS x, y, z FROM T GROUP BY y, z) AS T GROUP BY z;
