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]

This pull request has been approved by:

• HunDunDM
• nolouch

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[codecov]

Codecov Report

Merging #4219 (53bef99) into release-4.0 (b20ade2) will increase coverage by 0.56%.
The diff coverage is 93.18%.

@@               Coverage Diff               @@
##           release-4.0    #4219      +/-   ##
===============================================
+ Coverage        74.82%   75.39%   +0.56%     
===============================================
  Files              211      211              
  Lines            20061    20061              
===============================================
+ Hits             15011    15125     +114     
+ Misses            3717     3609     -108     
+ Partials          1333     1327       -6     

Flag	Coverage Δ
unittests	75.39% <93.18%> (+0.56%)	⬆️

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

Impacted Files	Coverage Δ
server/cluster/cluster.go	80.42% <0.00%> (+3.53%)	⬆️
server/core/storage.go	68.39% <75.00%> (+0.86%)	⬆️
server/region_syncer/client.go	79.68% <95.00%> (-2.67%)	⬇️
server/core/region_storage.go	80.23% <100.00%> (+1.75%)	⬆️
server/region_syncer/server.go	88.97% <100.00%> (+9.71%)	⬆️
server/schedulers/shuffle_hot_region.go	55.35% <0.00%> (-1.79%)	⬇️
server/member/lease.go	91.07% <0.00%> (-1.79%)	⬇️
server/grpc_service.go	58.66% <0.00%> (-1.21%)	⬇️
client/client.go	69.54% <0.00%> (-0.51%)	⬇️
... and 18 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: 7ed160f