store/tikv: increase batch split region limit (#24508)

[nolouch]

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

What problem does this PR solve?

Ref: #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?

How it Works: increase the split regions' limit.

Check List

Tests

• 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`
split single sql result: [map[SCATTER_FINISH_RATIO:1 TOTAL_SPLIT_REGION:2001]]
split all sql run total cost: 1m28.908304306s
split all sql run success!!!

Release note

• Increasing the split region limit makes the split table and pre split more stable

[ti-chi-bot]

@nolouch: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• disksing
• lhy1024

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[lhy1024]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 03df897

[nolouch]

/merge

[nolouch]

/run-unit-test

[nolouch]

/merge