txn: add a variable to control whether to lock unchanged unique keys (#44598)

[ti-chi-bot]

This is an automated cherry-pick of #44598

What problem does this PR solve?

Issue Number: close #44714

Problem Summary:

What is changed and how it works?

Add a session|global variable tidb_lock_unchanged_keys

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.

Add a `session|global` variable `tidb_lock_unchanged_keys`

[cfzjywxk]

Need to change the release-7.1.x document too,

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, zyguan

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-26 08:02:06.938841688 +0000 UTC m=+608292.340092139: ☑️ agreed by zyguan.
• 2023-06-26 08:08:25.782951401 +0000 UTC m=+608671.184201836: ☑️ agreed by cfzjywxk.

[ekexium]

/merge

[ti-chi-bot]

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