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 #3932 (025141e) into release-4.0 (2ddff54) will decrease coverage by 0.16%.
The diff coverage is 92.68%.

@@               Coverage Diff               @@
##           release-4.0    #3932      +/-   ##
===============================================
- Coverage        75.13%   74.96%   -0.17%     
===============================================
  Files              211      211              
  Lines            20026    20046      +20     
===============================================
- Hits             15046    15028      -18     
- Misses            3638     3691      +53     
+ Partials          1342     1327      -15     

Flag	Coverage Δ
unittests	74.96% <92.68%> (-0.17%)	⬇️

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

Impacted Files	Coverage Δ
server/region_syncer/client.go	77.94% <78.57%> (-5.53%)	⬇️
server/region_syncer/server.go	79.25% <83.33%> (-10.06%)	⬇️
server/core/region.go	92.38% <96.72%> (+0.64%)	⬆️
server/cluster/cluster.go	77.65% <100.00%> (-3.56%)	⬇️
server/statistics/store_collection.go	58.53% <0.00%> (-34.96%)	⬇️
server/id/id.go	76.19% <0.00%> (-4.77%)	⬇️
server/member/lease.go	91.07% <0.00%> (-1.79%)	⬇️
... and 13 more

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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: 025141e