## Proposed changes - Optimize `BlockingQueue`, `BlockingPriorityQueue` by swapping `notify` and `unlock` to reduce lock competition. Ref: https://www.boost.org/doc/libs/1_54_0/boost/thread/sync_bounded_queue.hpp - Change remote scan thread pool to `PriorityQueue`. ### Test result Before: ``` mysql> select sum(lo_partkey) from lineorder; +-----------------+ | sum(lo_partkey) | +-----------------+ | 300021444265405 | +-----------------+ 1 row in set (1.11 sec) ``` After: ``` mysql> select sum(lo_partkey) from lineorder; +-----------------+ | sum(lo_partkey) | +-----------------+ | 300021444265405 | +-----------------+ 1 row in set (0.80 sec) ```