Push down LIMIT When There Exists GROUP BY without ORDER BY or Any Aggregate Target

In a SELECT ... LIMIT, we don't push LIMIT down when the query has a GROUP BY but no ORDER BY.

Example Query:
`SELECT repo_id FROM github_events_hash GROUP BY repo_id LIMIT 10;`

Statement on Shards:
`COPY (SELECT repo_id FROM github_events_hash_102016 github_events_hash WHERE true GROUP BY repo_id) TO STDOUT`

We can extend the case where we push the LIMIT down if:
- There is a GROUP BY
- There is no ORDER BY
- There is no Aggregate Target