ddl: fix DDL still blocks if PD down and restart (#44295)

[ti-chi-bot]

This is an automated cherry-pick of #44295

What problem does this PR solve?

Issue Number: close #44267

Problem Summary:
It may clean the DML info at ETCD while leaving the MDL info at mysql.tidb_mdl_info.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  Run the schrDDL test on a cluster. Kill and restart PD, then check if DDL is blocked.
• 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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei

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[hawkingrei]

/lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, tangenta

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-07 05:50:21.762529834 +0000 UTC m=+51913.532868548: ☑️ agreed by tangenta.
• 2023-07-07 05:51:20.899471931 +0000 UTC m=+51972.669810643: ☑️ agreed by hawkingrei.

[tiancaiamao]

/merge

[ti-chi-bot]

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[purelind]

/test all

[tiprow]

@purelind: No presubmit jobs available for pingcap/tidb@release-7.1

In response to this:

/test all

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