lightning: Fix lints for lightning (#766)

[ti-srebot]

cherry-pick #766 to release-5.0
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# In br repo:
git pr https://github.com/pingcap/br/pull/989

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git push git@github.com:ti-srebot/br.git pr/989:release-5.0-4c77b100913e

---

What problem does this PR solve?

CI for tidb-lightning doesn't include lints, after merged into this repository, we hope it can pass the CI tests too.

What is changed and how it works?

Refactoring following what the linter says.

NOTEs

Those linters are disabled:

• errorlint, because pingcap/errors is incompatible with std errors.(see below)
  Standard errors uses err.Unwrap() to get cause of errors. But pingcap/errors uses err.Cause() and errors.Unwrap(err). If we follow this linter, using errors.Is to check error class, causes of errors would be omitted.
• wrapcheck, because there are too many unwrapped errors in tidb-lightning. An option is using a script to wrap them with errors.Trace(err), which is tricky, and in someway not really wrapped those errors. Maybe we need farther refactoring for error returning.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Release Note

• No release note

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: PR needs rebase.

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[YuJuncen]

/cherry-pick-invite

[ti-srebot]

@YuJuncen please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/br/invitations