statistics: update some vars for internal sessions correctly (#53977)

[ti-chi-bot]

This is an automated cherry-pick of #53977

What problem does this PR solve?

Issue Number: close #53972

Problem Summary:

What changed and how does it work?

We need to always update these vars for internal sessions. Otherwise, it won't work. And we will use the wrong default value.

See more at the issue.

Check List

Tests

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

  • I checked and no code files have been changed.

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.

修复 tidb_enable_async_merge_global_stats 和 tidb_analyze_partition_concurrency 在自动分析无法生效的问题
Fix tidb_enable_async_merge_global_stats and tidb_analyze_partition_concurrency not working in auto-analysis

[ti-chi-bot]

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idc-jenkins-ci-tidb/unit-test	c30c010	link	true	/test unit-test
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[fixdb]

@hi-rustin Do we really need it for 7.1?