#69 Use FQS mechanism for subquery planning

[Ashutosh Bapat]

Right now, if the whole query is shippable we use FQS to ship the whole query. If there are subqueries which are pushable and not the whole query, those are still planned by standard_planner. Instead, if a subquery is shippable, and not the whole query, we should use FQS planner to ship the query to the datanode, instead of using the standard planner.

[Koichi Suzuki]

3Q or 4Q, TBD