Rawkv cannot be restored through br in a large regions

[tikv-rawkv]

Feature Request

Is your feature request related to a problem? Please describe:

The br recovery tool tries to merge regions after splitting and then imports s3 data for recovery. However, after splitting, the regions are only on 3 nodes, and the space of other nodes cannot be used, causing the disk to be full.

Describe the feature you'd like:

After splitting, you need to rebalance to ensure that the region is even, and then import the data.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[Smityz]

When data already exists in the backup cluster, we have also found similar situations occurring. Currently, we have not identified where the problem is coming from.