store/tikv: pipe the split and scatter task

[nolouch]

Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

close #22969

Problem Summary:
Splitting and Scattering at the same time will cause multiple conflicts on the same region.

What is changed and how it works?

pipeline all splits task and scatter tasks

Release note

• Make the time consuming of split table more stable

I have run multiple times  to split 4000 regions, and the time is stale about `1m30s`
2021/03/17 17:25:57 split single sql result: [map[SCATTER_FINISH_RATIO:1 TOTAL_SPLIT_REGION:2001]]
2021/03/17 17:25:57 split all sql run total cost: 1m28.908304306s
2021/03/17 17:25:57 split all sql run success!!!

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[crazycs520]

Suggested change

	for _, r := range spResp.Regions {
	tid := int64(0)
	if tableID != nil {
	tid = *tableID
	}
	tid := int64(0)
	if tableID != nil {
	tid = *tableID
	}
	for _, r := range spResp.Regions {

[crazycs520]

Suggested change

	tid := int64(0)
	if tableID != nil {
	tid = *tableID
	}

[jackysp]

So it's still using

tidb/store/tikv/split_region.go

Line 217 in 579421f

_, err := s.pdClient.ScatterRegions(bo.ctx, []uint64{regionID}, opts...)

the batch API to scatter regions one by one. Which may make the scatter not work?

[jackysp]

I mean #22788 .
OK, I'm off topic.

[Yisaer]

The current region scatter may not work in a specific case which described in tikv/pd#3422 , I think it's not related to the api.

[ti-chi-bot]

@nolouch: PR needs rebase.

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