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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[disksing]

/run-all-tests

[codecov]

Codecov Report

Merging #4367 (fa0de44) into release-5.0 (581ee38) will increase coverage by 0.12%.
The diff coverage is 62.50%.

@@               Coverage Diff               @@
##           release-5.0    #4367      +/-   ##
===============================================
+ Coverage        74.76%   74.89%   +0.12%     
===============================================
  Files              245      245              
  Lines            24356    24370      +14     
===============================================
+ Hits             18210    18251      +41     
+ Misses            4522     4501      -21     
+ Partials          1624     1618       -6     

Flag	Coverage Δ
unittests	74.89% <62.50%> (+0.12%)	⬆️

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

Impacted Files	Coverage Δ
server/schedule/operator/step.go	68.45% <60.00%> (-0.20%)	⬇️
server/schedule/operator_controller.go	82.60% <100.00%> (ø)
pkg/etcdutil/etcdutil.go	84.70% <0.00%> (-3.53%)	⬇️
server/election/leadership.go	83.52% <0.00%> (-2.36%)	⬇️
server/tso/global_allocator.go	71.01% <0.00%> (-1.45%)	⬇️
server/member/member.go	65.21% <0.00%> (-1.09%)	⬇️
server/grpc_service.go	46.60% <0.00%> (-0.57%)	⬇️
server/tso/tso.go	70.55% <0.00%> (+0.61%)	⬆️
server/encryptionkm/key_manager.go	73.60% <0.00%> (+0.79%)	⬆️
... and 5 more

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