Support limit pushdown

[davidpsv17]

In some cases after grouping by certain column(s) ClickHouse still might return a lot of rows, so performing LIMIT on clickhouse side will be beneficial.

EXPLAIN VERBOSE select city from t1 LIMIT 10; QUERY PLAN

Limit (cost=0.00..0.00 rows=1 width=32) Output: city -> Foreign Scan on public.t1 (cost=0.00..0.00 rows=0 width=32) Output: city Remote SQL: SELECT city FROM "default".t1 (5 rows)