lightning: always get latest PD leader when access PD after initialized (#46726)

[ti-chi-bot]

This is an automated cherry-pick of #46726

What problem does this PR solve?

Issue Number: close #46688 close #43436

Problem Summary:

What is changed and how it works?

PD nodes may change, so lightning should always try to use latest informtion rather than the addresses in task configuration.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[lance6716]

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[purelind]

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[lance6716]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@4350f8d). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-6.5     #46757   +/-   ##
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  Lines                  ?     348268           
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  Hits                   ?     256532           
  Misses                 ?      75255           
  Partials               ?      16481           

[lance6716]

/test pull-br-integration-test

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@lance6716: No presubmit jobs available for pingcap/tidb@release-6.5

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[D3Hunter]

/lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: D3Hunter, lance6716

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-11 07:21:02.744780319 +0000 UTC m=+253844.669336713: ☑️ agreed by lance6716.
• 2023-09-11 07:24:58.582877347 +0000 UTC m=+254080.507433742: ☑️ agreed by D3Hunter.

[Benjamin2037]

LGTM