relay: reduce gtid clone for relay reader

[GMHDBJD]

What problem does this PR solve?

Issue Number: ref #4287

What is changed and how it works?

see go-mysql-org/go-mysql#746
reduce the clone for gtid

Check List

Tests

• Unit test
• Integration test

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

None

[ti-chi-bot]

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[ti-chi-bot]

@GMHDBJD: 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-dm-compatibility-test	a13b998	link	true	/test dm-compatibility-test
jenkins-ticdc/verify	a13b998	link	true	/test verify

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[ti-chi-bot]

PR needs rebase.

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