Unlimited high concurrent access to TiFlash may make TiFlash crash when using dynamic partition prune mode

## Enhancement

In TiDB, when dynamic partition prune mode is enabled, and if there is a full table scan for a partition table in TiFlash, assuming the partition table has n partitions, and if TiDB access TiFlash in cop mode, then the accessing concurrency will be
`n * dist_scan_concurrency`
That is to say the accessing concurrency will increase linearly if the total partition number increase. These unlimited high concurrent access to TiFlash may make TiFlash crash, we need find a way to limit the overall accessing concurrency.