diff: add a config for checkpointDB connection limit

[lance6716]

What problem does this PR solve?

if there're many check-threads that update checkpoint, default limit of 1 will cause pending and timeout

What is changed and how it works?

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

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

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

[ti-chi-bot]

@lance6716: PR needs rebase.

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[CLAassistant]

All committers have signed the CLA.