backend/local: add encode for retry scan region keys

[glorv]

What problem does this PR solve?

Fix the bug that the retry split key is not encoded, which causes scanned regions are incorrect.

What is changed and how it works?

• Fix the retry split scan keys bug
• Add more error checks and logs
• Add some unit tests

Check List

Tests

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

Side effects

Related changes

Release Note

• Fix the bug that the retry scan region key is not encoded, which causes the retryed region is not split.

[glorv]

/run-all-tests

[glorv]

/run-all-tests

[ti-srebot]

@Little-Wallace, Thanks for your review. The bot only counts LGTMs from Reviewers and higher roles, but you're still welcome to leave your comments. See the corresponding SIG page for more information. Related SIG: migrate(slack).