coordinator: use a healthy region count to start coordinator (#7044)

[ti-chi-bot]

This is an automated cherry-pick of #7044

What problem does this PR solve?

Issue Number: Close #6988 ,Close #7016

What is changed and how does it work?

• different source of region, regard FromSync and FromStorage as the stale region
• add a stale region count in regionTree to start coordinator first.
• atomic add/sub stale regions number by region updated

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Release note

use a healthy region count to avoid starting coordinator timeout.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• disksing
• rleungx

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5a95241) 75.62% compared to head (15d9a75) 75.51%.
Report is 1 commits behind head on release-6.5.

Files	Patch %	Lines
server/cluster/cluster.go	0.00%	0 Missing and 1 partial ⚠️
server/region_syncer/client.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-6.5    #7363      +/-   ##
===============================================
- Coverage        75.62%   75.51%   -0.12%     
===============================================
  Files              332      332              
  Lines            33721    33743      +22     
===============================================
- Hits             25502    25480      -22     
- Misses            6032     6062      +30     
- Partials          2187     2201      +14     

Flag	Coverage Δ
unittests	75.51% <94.44%> (-0.12%)	⬇️

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[bufferflies]

/merge

[ti-chi-bot]

@bufferflies: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: 39070c4

[bufferflies]

/merge

[ti-chi-bot]

@bufferflies: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[bufferflies]

/merge

[ti-chi-bot]

@bufferflies: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[bufferflies]

/rebuild

[HuSharp]

/retest