tikv: increase batch split region limit

[nolouch]

Signed-off-by: nolouch [email protected]

What problem does this PR solve?

Ref: pingcap/tidb#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 in 4.0.

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