owner: fix new owner updating checkpoint too early with pending DDL (#2252)

[ti-chi-bot]

This is an automated cherry-pick of #2252

What problem does this PR solve?

• DDL lost after owner is killed (kill_owner_with_ddl fails) #1260 was reintroduced into the new owner (released in v5.1.0)

What is changed and how it works?

• Changed the start-ts of DDL puller to checkpoint - 1, to satisfy the open-interval assumption by TiKV.
• Fetch the snapshot of meta data at checkpoint instead of checkpoint-1 avoid overlap between listened DDL events and fetched schema snapshot.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix potential DDL loss when owner crashes while executing DDL

[ti-chi-bot]

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[amyangfei]

/merge

[liuzix]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: bddbe00

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.1@cae9441). Click here to learn what that means.
The diff coverage is n/a.

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