Compatible batch copr with load-based replica read

[you06]

Enhancement

We have 2 optimizations which improve the performance of distsql read, one is the batch coprocessor and another is the load-based replica read. But they are currently not compatible, because sending batched tasks to the replica store will produce region misses.

tidb/store/copr/coprocessor.go

Lines 1126 to 1132 in fa28db1

	// TODO: Load-based replica read is currently not compatible with store batched tasks now.
	// The batched tasks should be dispatched to their own followers, but it's not implemented yet.
	// So, only enable load-based replica read when there is no batched tasks.
	var busyThresholdMs uint32
	if len(copReq.Tasks) == 0 {
	busyThresholdMs = uint32(worker.req.StoreBusyThreshold.Milliseconds())
	}

As an enhancement, the batched task should be re-split before being sent to the replica nodes. It might choose the best replica node in the coprocessor client.