ddl: make ToProxyJob return value to avoid heap allocation

[tangenta]

What problem does this PR solve?

Issue Number: ref #14766

Problem Summary:

What is changed and how it works?

• Makes ToProxyJob return a value instead of a pointer.
• Clone the job in the failpoint mockParallelSameDDLJobTwice to prevent sharing the same instance(which will fail in concurrent DDL implementation).

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• xiongjiwei

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[hawkingrei]

/run-all-tests

[tangenta]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 0883004

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/088300469257f68beb9a685a99c18eba19a826b2