sorter: fix Unified Sorter error handling & add more unit test cases (#1619)

[ti-srebot]

cherry-pick #1619 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In ticdc repo:
git pr https://github.com/pingcap/ticdc/pull/1677

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/ticdc.git pr/1677:release-4.0-22774319c392

---

What problem does this PR solve?

• Unified Sorter might return an unhelpful error ErrWorkerPoolHandleCancelled for some IO errors.
• Some error handling code might cause data race.

What is changed and how it works?

• Filter the ErrWorkerPoolHandleCancelled error.
• Add appropriate synchronization to avoid races.
• Add unit test cases for these fixes.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix Unified Sorter returning unhelpful error messages & other misc. fixes.

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-chi-bot]

@ti-srebot: PR needs rebase.

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[amyangfei]

cherry-picked in #1730