scheduler: fix scatter range scheduler limit (#3361)

[ti-srebot]

cherry-pick #3361 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In pd repo:
git pr https://github.com/tikv/pd/pull/3401

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/pd.git pr/3401:release-4.0-680a3a48d3cd

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What problem does this PR solve?

Closes #3356

What is changed and how it works?

This PR fixes the limit constriction for scatter range scheduler.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix the limit constriction of the scatter range scheduler

[ti-chi-bot]

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[ti-srebot]

@rleungx you're already a collaborator in bot's repo.

[HunDunDM]

@rleungx please fix the conflict.

[rleungx]

/run-all-tests

[codecov]

Codecov Report

Merging #3401 (2417ff1) into release-4.0 (0899703) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-4.0    #3401      +/-   ##
===============================================
- Coverage        74.74%   74.70%   -0.05%     
===============================================
  Files              209      209              
  Lines            19752    19760       +8     
===============================================
- Hits             14764    14761       -3     
- Misses            3653     3658       +5     
- Partials          1335     1341       +6     

Flag	Coverage Δ
unittests	74.70% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/schedulers/scatter_range.go	80.79% <100.00%> (+1.07%)	⬆️
pkg/etcdutil/etcdutil.go	72.05% <0.00%> (-10.30%)	⬇️
server/heartbeat_streams.go	65.93% <0.00%> (-6.60%)	⬇️
server/region_syncer/client.go	78.74% <0.00%> (-4.73%)	⬇️
server/tso/tso.go	81.15% <0.00%> (-2.90%)	⬇️
server/member/leader.go	66.05% <0.00%> (-2.30%)	⬇️
server/grpc_service.go	59.87% <0.00%> (-0.41%)	⬇️
server/server.go	74.66% <0.00%> (+1.32%)	⬆️
pkg/mock/mockhbstream/mockhbstream.go	88.88% <0.00%> (+1.85%)	⬆️
pkg/dashboard/adapter/manager.go	81.52% <0.00%> (+4.34%)	⬆️
... and 2 more

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[HunDunDM]

/merge

[ti-chi-bot]

@HunDunDM: 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: 2417ff1