Pushdown the limit if possible(group by partition column); Fix the queri...

...es that should not push down limit
PlanNode Explain string for the query:
RETURN RESULTS TO STORED PROCEDURE
 LIMIT 1
  ORDER BY (SORT)
   RECEIVE FROM ALL PARTITIONS

PlanNode Explain string:
RETURN RESULTS TO STORED PROCEDURE
 LIMIT 1
  ORDER BY (SORT)
   INDEX SCAN of "T3" using its primary key index (for deterministic order only)
   inline Hash AGGREGATION ops: COUNT(

Partition 1: return row (3, 1) to coordinator. Partition 2: return row (2, 2) to coordinator. 
Coordinator apply order by, limit and return row (3, 1).

Why not OK?