planner: simplify StatsCacheWrapper

[qw4990]

What problem does this PR solve?

Issue Number: ref #44461

Problem Summary: planner: simplify StatsCacheWrapper

What is changed and how it works?

planner: simplify StatsCacheWrapper

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

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-10 03:33:14.180867359 +0000 UTC m=+302885.951206070: ☑️ agreed by hawkingrei.
• 2023-07-10 03:35:54.680429179 +0000 UTC m=+303046.450767892: ☑️ agreed by chrysan.