lightning/backend: use optimistic transaction for tidb backend

[jackysp]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:
Lightning tidb backend doesn't use optimistic transaction which may cause many useless pessimistic requests to TiKV.

What is changed and how it works?

Set optimistic transaction for it.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1. sysbench prepare a 10000000 rows table
2. dump the table
3. truncate sbtest
4. lightning tidb backend load the table

• 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

lightning/backend: use optimistic transaction for tidb backend

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• kennytm

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/d7b7cfe532ef048d03ab762733b31e70e2c8f581

[glorv]

/merge

[ti-chi-bot]

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

Commit hash: 76d56aa

[ti-chi-bot]

@jackysp: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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