*: fix race in hot region config (#4167)

[HunDunDM]

Signed-off-by: HunDunDM hundundm@gmail.com

This is a cherry-pick of #4167

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]

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• lhy1024
• rleungx

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[codecov]

Codecov Report

Merging #4176 (df779c3) into release-5.1-20210926 (15cf76e) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@                   Coverage Diff                    @@
##           release-5.1-20210926    #4176      +/-   ##
========================================================
+ Coverage                 75.15%   75.22%   +0.06%     
========================================================
  Files                       245      245              
  Lines                     24863    24863              
========================================================
+ Hits                      18687    18702      +15     
+ Misses                     4541     4532       -9     
+ Partials                   1635     1629       -6     

Flag	Coverage Δ
unittests	75.22% <100.00%> (+0.06%)	⬆️

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	83.65% <100.00%> (ø)
server/tso/global_allocator.go	70.55% <0.00%> (-5.00%)	⬇️
server/schedulers/random_merge.go	60.65% <0.00%> (-3.28%)	⬇️
server/member/member.go	67.20% <0.00%> (-2.69%)	⬇️
server/election/leadership.go	81.17% <0.00%> (-2.36%)	⬇️
server/tso/allocator_manager.go	78.63% <0.00%> (-0.51%)	⬇️
server/grpc_service.go	48.11% <0.00%> (-0.46%)	⬇️
client/client.go	71.96% <0.00%> (+0.27%)	⬆️
server/cluster/cluster.go	83.60% <0.00%> (+0.34%)	⬆️
server/cluster/coordinator.go	74.05% <0.00%> (+0.70%)	⬆️
... and 5 more

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