cluster: add a switch about region flow updates (#2844)

[ti-srebot]

cherry-pick #2844 to release-4.0

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Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

ref #2843
reduce the updates with a switch.

What is changed and how it works?

Check List

Tests

• Unit test

Release note

• add a switch about region flow updates in region heartbeat

[ti-srebot]

@nolouch please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/pd/invitations

[HunDunDM]

LGTM

[codecov]

Codecov Report

Merging #2848 into release-4.0 will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-4.0    #2848      +/-   ##
===============================================
+ Coverage        77.31%   77.49%   +0.18%     
===============================================
  Files              208      208              
  Lines            22765    22768       +3     
===============================================
+ Hits             17601    17645      +44     
+ Misses            3829     3804      -25     
+ Partials          1335     1319      -16     

Impacted Files	Coverage Δ
server/cluster/cluster.go	80.91% <100.00%> (+0.01%)	⬆️
server/config/config.go	77.65% <100.00%> (+0.08%)	⬆️
pkg/btree/btree.go	86.84% <0.00%> (-0.81%)	⬇️
server/grpc_service.go	59.88% <0.00%> (-0.20%)	⬇️
server/server.go	76.96% <0.00%> (+0.44%)	⬆️
server/schedulers/adjacent_region.go	76.59% <0.00%> (+1.06%)	⬆️
server/cluster/coordinator.go	71.22% <0.00%> (+1.19%)	⬆️
server/member/leader.go	73.54% <0.00%> (+1.55%)	⬆️
server/kv/etcd_kv.go	88.31% <0.00%> (+3.89%)	⬆️
server/tso/tso.go	82.91% <0.00%> (+4.43%)	⬆️
... and 4 more

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[jebter]

/run-all-tests

[jebter]

/merge

[ti-srebot]

/run-all-tests