core: allow cancel load region (#4175)

[ti-chi-bot]

This is an automated cherry-pick of #4175

What problem does this PR solve?

Fix #3936

What is changed and how it works?

• add cancel support for core.LoadRegion functions
• replace chan bool with contextes in region syncer client
• add test case

Check List

Tests

• Unit test

Release note

Fix the issue that PD may not elect leader as soon as leader step down

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• nolouch

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[nolouch]

lgtm

[nolouch]

the statics check failed in test.

[disksing]

/merge

[ti-chi-bot]

@disksing: 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: b1ff18b

[disksing]

/run-all-tests

[ti-chi-bot]

[FORMAT CHECKER NOTIFICATION]

Please provide the associated issue number in the commit messages, for example: close #12345, or ref #12345.

[disksing]

/run-all-tests

[disksing]

/run-all-tests

[disksing]

/run-all-tests

[codecov]

Codecov Report

Merging #4218 (2538a29) into release-5.0 (e482eed) will increase coverage by 0.20%.
The diff coverage is 89.65%.

@@               Coverage Diff               @@
##           release-5.0    #4218      +/-   ##
===============================================
+ Coverage        74.78%   74.98%   +0.20%     
===============================================
  Files              245      245              
  Lines            24370    24362       -8     
===============================================
+ Hits             18225    18268      +43     
+ Misses            4520     4489      -31     
+ Partials          1625     1605      -20     

Flag	Coverage Δ
unittests	74.98% <89.65%> (+0.20%)	⬆️

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

Impacted Files	Coverage Δ
server/cluster/cluster.go	82.92% <0.00%> (+0.90%)	⬆️
server/region_syncer/server.go	88.88% <ø> (+5.80%)	⬆️
server/core/storage.go	69.20% <75.00%> (+1.52%)	⬆️
server/region_syncer/client.go	79.68% <94.11%> (-2.01%)	⬇️
server/core/region_storage.go	77.65% <100.00%> (+1.79%)	⬆️
server/schedulers/shuffle_hot_region.go	55.10% <0.00%> (-10.21%)	⬇️
server/election/leadership.go	83.52% <0.00%> (-3.53%)	⬇️
server/schedulers/random_merge.go	60.00% <0.00%> (-3.34%)	⬇️
server/grpc_service.go	46.94% <0.00%> (-0.23%)	⬇️
... and 10 more

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