make async commit configurable (example pr)

[guoshouyan]

What problem does this PR solve?

this is an example pr to demonstrate my idea. Will add more detail and test later

during our test on stale read on tidb, we find out that there's unbalance read traffic to some of the tikv nodes. During our investigation, we find out that i's async_commit will cause the lagging of resolved_ts. Before we make stale read more stable, I'm looking for adding an option in lightning to disable the async_commit

Issue Number: close #xxx

Problem Summary:

What is changed and how it works?

Check List

Tests

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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This pull request is closed because its related version has closed automatic cherry-picking.
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see:

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https://book.prow.tidb.net/#/plugins/cherrypicker