br: less split during restoring

[YuJuncen]

Port of pingcap/br#1377

What problem does this PR solve?

Partially fix pingcap/br#1374

What is changed and how it works?

before：
--|-------t1 data-------|-----|---t2 data-------|
after: 
----------t1 data-------|---------t2 data-------|

Legends:
'|' the split point
'-' the key space

Also, downloadSST would find rewrite rules by file instead of region start key for now.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  The same case as Too many empty region after restoration of many small tables br#1374, the region count reduced about 1/2.

Release note

BR would split less regions to reduce the number of empty region after restoration.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• kennytm

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[YuJuncen]

/sig migrate
/component br

[YuJuncen]

/hold because the random download file skipped warnings. (Even didn't made data lost)

[YuJuncen]

/unhold because:

1. The download file skipped only happens in v4.x, and current release-4.0 has the same problem.
2. The probability of scattering no-empty region could be omitted via one less scatter.

[YuJuncen]

/remove-hold
🤔

[YuJuncen]

/merge

[ti-chi-bot]

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

Commit hash: 2dad755

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