*: fix race in hot region config

[rleungx]

What problem does this PR solve?

Close #4159.

What is changed and how it works?

Use the lock correctly.

Check List

Tests

• Unit test

Release note

Fix the data race problem of hot region config

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• lhy1024

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[rleungx]

/run-unit-test

[codecov]

Codecov Report

Merging #4167 (962cace) into master (455a2f9) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4167      +/-   ##
==========================================
+ Coverage   74.59%   74.63%   +0.03%     
==========================================
  Files         260      260              
  Lines       26479    26481       +2     
==========================================
+ Hits        19753    19764      +11     
+ Misses       4945     4940       -5     
+ Partials     1781     1777       -4     

Flag	Coverage Δ
unittests	74.63% <100.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
server/schedulers/hot_region_config.go	92.09% <100.00%> (+0.09%)	⬆️
server/kv/etcd_kv.go	69.56% <0.00%> (-10.15%)	⬇️
server/region_syncer/client.go	77.46% <0.00%> (-4.23%)	⬇️
server/tso/tso.go	61.93% <0.00%> (-2.28%)	⬇️
server/server.go	70.35% <0.00%> (-1.05%)	⬇️
server/election/leadership.go	77.31% <0.00%> (-1.04%)	⬇️
server/grpc_service.go	46.37% <0.00%> (-0.69%)	⬇️
client/base_client.go	81.52% <0.00%> (-0.55%)	⬇️
client/client.go	71.94% <0.00%> (-0.14%)	⬇️
server/tso/allocator_manager.go	65.16% <0.00%> (+0.33%)	⬆️
... and 9 more

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[rleungx]

/merge

[ti-chi-bot]

@rleungx: 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: 962cace

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4170.