pkg/lightning: make save file checkpoint more compatible to avoid checkpoint data loss (#885)

[ti-srebot]

cherry-pick #885 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/889

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

git push [email protected]:ti-srebot/br.git pr/889:release-4.0-d95f9fdfcd29

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What problem does this PR solve?

Currently, lightning use ioutil.WriteFile to save checkpoint file, which can lead to data loss if lightning exit before the file is fully writen.

What is changed and how it works?

First write checkpoint content to a temp file and then replace it to the target file.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release Note

• Fix a bug that lightning unexpected exit may cause checkpoint file truncated to 0 sized.

[kennytm]

/lgtm

[lance6716]

/lgtm

[kennytm]

/run-integration-test