restore: elision of scattering when max-replica -ge store-count

[YuJuncen]

What problem does this PR solve?

Issue Number: close #27425

Problem Summary: After retrying failed scatter requests, when max-replica > store-number, it would always fail. A full retry costs about 10+ seconds, which is unacceptable for upon thousands even millions of regions.

What is changed and how it works?

Proposal: xxx

What's Changed:
When max-replica >= store-number, skip scattering, since it is no meaning to scatter in this condition.

How it Works:
So, the retry never happen.

Check List

Tests

• Manual test (add detailed scripts or steps below)

In a typical TiUP playground (store-count = 1, max-replica = 3):

In a testing cluster (store-count = 3, max-replica = 1):

Side effects

• Performance regression: Consumes more Networking (O(1))

Release note

Fix a bug that causes BR get stuck in some of small clusters.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• kennytm

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[YuJuncen]

/cc kennytm
/cc 3pointer
/sig migrate
/component br

[YuJuncen]

/run-integration-tests

[kennytm]

when max-replica == store-count > 1, without scattering there will be a large leader imbalance (though size is balanced). not sure if PD can quickly fix it afterwards.

maybe just skip scatter when max-replica > store-count || store-count = 1?

[kennytm]

rest LGTM

[YuJuncen]

/merge

[ti-chi-bot]

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

Commit hash: e88bc45

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