schedule: fix race for region scatter (#3760)

[ti-chi-bot]

This is an automated cherry-pick of #3760

Signed-off-by: yisaer disxiaofei@163.com

What problem does this PR solve?

As totalCountByStore calling the function getDistributionByGroupLocked, this function should be procteded by rw mutex otherwise it will cause data race panic

What is changed and how it works?

Add mutex to protect it.

Check List

Tests

• Unit test

Release note

Fix pd may get panic during scattering region

[codecov]

Codecov Report

Merging #3762 (83435fc) into release-5.0 (f4d1a86) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-5.0    #3762      +/-   ##
===============================================
+ Coverage        74.61%   74.66%   +0.05%     
===============================================
  Files              245      245              
  Lines            24183    24185       +2     
===============================================
+ Hits             18043    18057      +14     
+ Misses            4521     4514       -7     
+ Partials          1619     1614       -5     

Flag	Coverage Δ
unittests	74.66% <100.00%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
server/schedule/region_scatterer.go	88.47% <100.00%> (+0.10%)	⬆️
server/schedulers/shuffle_hot_region.go	54.73% <0.00%> (-9.48%)	⬇️
server/tso/local_allocator.go	73.61% <0.00%> (-6.95%)	⬇️
server/election/lease.go	89.83% <0.00%> (-5.09%)	⬇️
tools/pd-ctl/pdctl/command/operator.go	65.99% <0.00%> (-1.22%)	⬇️
server/server.go	73.38% <0.00%> (-0.91%)	⬇️
server/config/persist_options.go	91.33% <0.00%> (-0.79%)	⬇️
server/handler.go	41.97% <0.00%> (-0.50%)	⬇️
client/client.go	72.24% <0.00%> (+0.13%)	⬆️
client/base_client.go	85.94% <0.00%> (+0.54%)	⬆️
... and 8 more

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• nolouch
• rleungx

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: 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: 3521301