region_syncer: reduce saveKV of client (#3808)

[ti-chi-bot]

This is an automated cherry-pick of #3808

What problem does this PR solve?

What is changed and how it works?

• By introducing part of the heartbeat processing logic in the client of region_syncer, unnecessary saveKV operations are reduced.

Check List

Tests

• Unit test

Code changes

Side effects

Related changes

Release note

Improved the performance of synchronizing Region information between PDs.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• HunDunDM
• nolouch

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[codecov]

Codecov Report

Merging #3933 (cc28564) into release-5.1 (59693ab) will increase coverage by 0.04%.
The diff coverage is 92.50%.

@@               Coverage Diff               @@
##           release-5.1    #3933      +/-   ##
===============================================
+ Coverage        75.08%   75.13%   +0.04%     
===============================================
  Files              245      245              
  Lines            24614    24634      +20     
===============================================
+ Hits             18482    18508      +26     
+ Misses            4513     4506       -7     
- Partials          1619     1620       +1     

Flag	Coverage Δ
unittests	75.13% <92.50%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
server/region_syncer/client.go	81.69% <78.57%> (-1.02%)	⬇️
server/region_syncer/server.go	88.97% <83.33%> (+5.63%)	⬆️
server/core/region.go	91.20% <96.61%> (+0.78%)	⬆️
server/cluster/cluster.go	82.39% <100.00%> (-0.49%)	⬇️
pkg/etcdutil/etcdutil.go	84.70% <0.00%> (-3.53%)	⬇️
server/member/member.go	65.59% <0.00%> (-3.23%)	⬇️
pkg/dashboard/adapter/manager.go	79.78% <0.00%> (-3.20%)	⬇️
server/schedule/operator/step.go	67.44% <0.00%> (-1.67%)	⬇️
server/encryptionkm/key_manager.go	71.78% <0.00%> (-1.66%)	⬇️
server/server.go	71.58% <0.00%> (ø)
... and 7 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: cc28564