Enhancement

TiDB support partition table, and for a partition table with n partitions, when in MPP mode, for each partition, TiDB will generate 1 mpp task, so the mpp task number will explode if the partition table contains too many partitions. In order to reduce the mpp task number, a new executor named PartitionTableScan is introduced in tipb, both TiFlash and TiDB need to adapt this change.