refactor: simplify 2-stage, remove duplicates

- Refactor 2-stage aggregation pipeline, so it is easier to read and work with
- add logic for removing duplicate grouping expressions and aggregates from local stage:
 
`select sum(a), sum(a) from t` 

We should compute only one aggregate at local stage.

The same goes for grouping expressions: 

`select sum(distinct a) from t group by a`

At local stage we must use `a` only once in projection.