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:

• nolouch

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[HunDunDM]

/rebuild

[HunDunDM]

/rebuild

[codecov]

Codecov Report

Merging #3993 (d1d1ae8) into release-5.0 (88726b2) will decrease coverage by 0.22%.
The diff coverage is 92.68%.

@@               Coverage Diff               @@
##           release-5.0    #3993      +/-   ##
===============================================
- Coverage        74.89%   74.67%   -0.23%     
===============================================
  Files              245      245              
  Lines            24256    24276      +20     
===============================================
- Hits             18167    18128      -39     
- Misses            4485     4529      +44     
- Partials          1604     1619      +15     

Flag	Coverage Δ
unittests	74.67% <92.68%> (-0.23%)	⬇️

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	90.61% <96.72%> (+0.86%)	⬆️
server/cluster/cluster.go	82.40% <100.00%> (-0.91%)	⬇️
server/schedulers/shuffle_hot_region.go	55.10% <0.00%> (-10.21%)	⬇️
server/election/leadership.go	83.52% <0.00%> (-5.89%)	⬇️
server/tso/tso.go	68.71% <0.00%> (-5.53%)	⬇️
server/election/lease.go	89.83% <0.00%> (-5.09%)	⬇️
server/member/member.go	66.84% <0.00%> (-4.35%)	⬇️
pkg/dashboard/adapter/manager.go	79.78% <0.00%> (-3.20%)	⬇️
... and 10 more

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[nolouch]

PTAL @rleungx

[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: acc28bf