operator: check store status for running operators (#4223)

[ti-chi-bot]

This is an automated cherry-pick of #4223

What problem does this PR solve?

Fix #3353

What is changed and how it works?

• For TransferLeader, AddLearner, AddPeer, check if the target store is down and cancel the operator if need.
• Other types of OperatorStep may or may not get blocked by down store, we don't restrict them in the PR.
• Add tests

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

Fix the issue that operator can get blocked due to down store

[ti-chi-bot]

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[codecov]

Codecov Report

Merging #4365 (b00c496) into release-5.2 (095ab85) will decrease coverage by 0.04%.
The diff coverage is 62.50%.

@@               Coverage Diff               @@
##           release-5.2    #4365      +/-   ##
===============================================
- Coverage        74.87%   74.83%   -0.05%     
===============================================
  Files              249      249              
  Lines            25672    25686      +14     
===============================================
  Hits             19223    19223              
- Misses            4772     4782      +10     
- Partials          1677     1681       +4     

Flag	Coverage Δ
unittests	74.83% <62.50%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
server/schedule/operator/step.go	68.25% <60.00%> (-1.52%)	⬇️
server/schedule/operator_controller.go	83.56% <100.00%> (ø)
server/schedulers/shuffle_hot_region.go	55.55% <0.00%> (-10.11%)	⬇️
server/region_syncer/client.go	75.00% <0.00%> (-8.60%)	⬇️
client/base_client.go	82.81% <0.00%> (-2.09%)	⬇️
server/server.go	72.18% <0.00%> (-1.36%)	⬇️
server/grpc_service.go	47.45% <0.00%> (ø)
server/cluster/coordinator.go	73.82% <0.00%> (+0.67%)	⬆️
server/tso/tso.go	65.74% <0.00%> (+1.10%)	⬆️
... and 4 more

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