store/copr: optimize copIterator by avoid start new goroutine[DNM]

[crazycs520]

What problem does this PR solve?

Issue Number: ref #56649

Problem Summary: optimize cop iterator by avoid start new goroutine

What changed and how does it work?

Before this PR, copIterator always start at least 1 copIteratorWorker goroutine and 1 copIteratorTaskSender goroutine. That's too wasteful for small queries.

This PR avoid start new goroutine for small queries, for better performance, following is the sysbench oltp_index_scan test, and the range size is 2:

Check List

Tests

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

  Refactor, without logical change.

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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[tiprow]

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[crazycs520]

/retest-required

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[crazycs520]

/retest-required

[tiprow]

@crazycs520: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/retest-required

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[codecov]

Codecov Report

Attention: Patch coverage is 72.90323% with 42 lines in your changes missing coverage. Please review.

Project coverage is 73.6251%. Comparing base (2a1f646) to head (53010ee).
Report is 5 commits behind head on master.

❗ Current head 53010ee differs from pull request most recent head 0339fd2

Please upload reports for the commit 0339fd2 to get more accurate results.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58140        +/-   ##
================================================
+ Coverage   73.2116%   73.6251%   +0.4134%     
================================================
  Files          1675       1676         +1     
  Lines        461921     462239       +318     
================================================
+ Hits         338180     340324      +2144     
+ Misses       102981     101156      -1825     
+ Partials      20760      20759         -1     

Flag	Coverage Δ
integration	43.1963% <72.9032%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	46.0028% <ø> (+0.0090%)	⬆️

[ti-chi-bot]

@crazycs520: 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
pull-mysql-client-test	0339fd2	link	true	/test pull-mysql-client-test
idc-jenkins-ci-tidb/mysql-test	0339fd2	link	true	/test mysql-test
idc-jenkins-ci-tidb/check_dev_2	0339fd2	link	true	/test check-dev2
idc-jenkins-ci-tidb/check_dev	0339fd2	link	true	/test check-dev
idc-jenkins-ci-tidb/unit-test	0339fd2	link	true	/test unit-test

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