leadership: check leadership key before exit watch loop

[lhy1024]

What problem does this PR solve?

Issue Number: Close #6977

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

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[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[rleungx]

Why open a new PR?

[lhy1024]

Why open a new PR?

To solve #6977, reduce unnecessary elections after stopping pd-leader-io-hang.

[codecov]

Codecov Report

Merging #6979 (7643816) into master (7dc9d17) will decrease coverage by 0.26%.
Report is 177 commits behind head on master.
The diff coverage is 83.01%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6979      +/-   ##
==========================================
- Coverage   74.51%   74.25%   -0.26%     
==========================================
  Files         427      433       +6     
  Lines       45149    45967     +818     
==========================================
+ Hits        33642    34135     +493     
- Misses       8552     8824     +272     
- Partials     2955     3008      +53     

Flag	Coverage Δ
unittests	74.25% <83.01%> (-0.26%)	⬇️

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[ti-chi-bot]

PR needs rebase.

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