dm/syncer : fix wrong causality key when schema has prefix index

[WizardXiao]

What problem does this PR solve?

fix wrong causality key when use prefix index. #3286

What is changed and how it works?

Causality key will use prefix index to advoid dmls divide into different work group. For example.
create table t (c1 text, c2 int unique, unique key c1(c1(3)));
insert into t values ('1234', 1) --> key is 123.c1.
insert into t values ('1235', 2) --> key is 123.c1.

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update key monitor metrics in both TiCDC document and official document

Release note

`support calculate causality key with prefix index`.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Ehco1996
• lichunzhu

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[WizardXiao]

/run-dm-integration-test

[WizardXiao]

/run-dm-compatibility-test

[WizardXiao]

/run-integration-test

[codecov-commenter]

Codecov Report

Merging #3467 (53bf372) into master (24030f8) will increase coverage by 0.9626%.
The diff coverage is 67.8657%.

@@               Coverage Diff                @@
##             master      #3467        +/-   ##
================================================
+ Coverage   56.5356%   57.4983%   +0.9626%     
================================================
  Files           211        233        +22     
  Lines         22798      23832      +1034     
================================================
+ Hits          12889      13703       +814     
- Misses         8598       8760       +162     
- Partials       1311       1369        +58     

[WizardXiao]

/run-kafka-integration-test

[Ehco1996]

/merge

[ti-chi-bot]

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

Commit hash: 53bf372

[WizardXiao]

/run-unit-test

[lance6716]

/run-kafka-integration-test