*: adjusts the merge sort overlap threshold/merge sort file count step based on concurrency (#62483)

[ti-chi-bot]

This is an automated cherry-pick of #62483

What problem does this PR solve?

Issue Number: close #60951

Problem Summary:
If the concurrency is less than 8, the memory used may be less than the real requirement.
By adjusting the threshold, we can reduce the actual memory requirement due to lower statistical bias.

What changed and how does it work?

set the threshold to 250*concurrency

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.

fix a global sort bug that may be blocked in CPU: Memory = 1:2 machine 

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

@wjhuang2016 This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	989609f	link	true	/test unit-test
idc-jenkins-ci-tidb/build	989609f	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	989609f	link	true	/test check-dev2
idc-jenkins-ci-tidb/check_dev	989609f	link	true	/test check-dev
idc-jenkins-ci-tidb/mysql-test	989609f	link	true	/test mysql-test

Full PR test history. Your PR dashboard.

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