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:

• nolouch

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[codecov]

Codecov Report

Merging #4217 (8b5ef1b) into release-5.1 (32885c6) will decrease coverage by 0.27%.
The diff coverage is 89.65%.

@@               Coverage Diff               @@
##           release-5.1    #4217      +/-   ##
===============================================
- Coverage        75.41%   75.13%   -0.28%     
===============================================
  Files              245      245              
  Lines            24718    24710       -8     
===============================================
- Hits             18642    18567      -75     
- Misses            4471     4515      +44     
- Partials          1605     1628      +23     

Flag	Coverage Δ
unittests	75.13% <89.65%> (-0.28%)	⬇️

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

Impacted Files	Coverage Δ
server/cluster/cluster.go	82.39% <0.00%> (-0.38%)	⬇️
server/region_syncer/server.go	82.96% <ø> (-6.01%)	⬇️
server/core/storage.go	68.44% <75.00%> (-0.77%)	⬇️
server/region_syncer/client.go	79.68% <94.11%> (-2.01%)	⬇️
server/core/region_storage.go	77.65% <100.00%> (+1.79%)	⬆️
server/member/member.go	67.20% <0.00%> (-5.92%)	⬇️
server/id/id.go	76.19% <0.00%> (-4.77%)	⬇️
pkg/dashboard/adapter/manager.go	78.72% <0.00%> (-4.26%)	⬇️
server/tso/allocator_manager.go	77.46% <0.00%> (-3.01%)	⬇️
... and 11 more

---

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[nolouch]

lgtm

[ti-chi-bot]

[FORMAT CHECKER NOTIFICATION]

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

[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

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 8b5ef1b

[disksing]

/run-all-tests