ddl: move preSplitAndScatter to ddl worker

[tangenta]

What problem does this PR solve?

Issue Number: close #44785

Problem Summary:

Before this PR, TiDB splits and scatters region after the DDL is complete. This may cause the issue that client waits a long time but the ddl info records only a few time.

What is changed and how it works?

As recommended in #44785 (comment), I moved preSplitAndScatter to DDL worker side.

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

[tiprow]

Hi @tangenta. Thanks for your PR.

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[zimulala]

Could we add manual test manual to the PR description?
Rest LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wjhuang2016, zimulala

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-10 03:59:15.898683096 +0000 UTC m=+304447.669021801: ☑️ agreed by zimulala.
• 2023-07-12 05:26:44.557041478 +0000 UTC m=+482496.327380191: ☑️ agreed by wjhuang2016.