sinkv2(cdc): add busy ratio metric for conflict detector

[hicqu]

What problem does this PR solve?

Issue Number: ref #5928

What is changed and how it works?

• Add a metrics for conflict detector background goroutine busy ratio
• Use slice instead of BTree for depender list, because all events are already sorted

Check List

Tests

• Unit test
• Manual bench
  With the patch : BenchmarkHighConflictsWithManyKeys-16 1230 855992 ns/op
Without the patch: BenchmarkHighConflictsWithManyKeys-16 1501 961344 ns/op

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

None

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[hicqu]

The PR seems not necessary. Test result shows the conflict detector background thread is always free enough.