ticdc: change the calculation of first sync point ts

[hongyunyan]

What problem does this PR solve?

Issue Number: close #11212

What is changed and how it works?

In this PR, we change the calculation of the first sync point ts.
As before, we use startTs as the first sync point ts.
Afer the PR, the first-sync-point-ts = k * sync-point-ts-interval,
and we will choose the minimal k that satisfy the first-sync-point-ts >= startTs.

Check List

Tests

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

Questions

Will it cause performance regression or break compatibility?

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

Release note

Change the calculation of first sync point ts.

[hongyunyan]

/retest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.6391%. Comparing base (667cea0) to head (cf76a07).
Report is 9 commits behind head on master.

Additional details and impacted files

Components	Coverage Δ
cdc	61.4440% <100.0000%> (-0.0670%)	⬇️
dm	51.2111% <ø> (+0.0100%)	⬆️
engine	63.3879% <ø> (-0.0142%)	⬇️

Flag	Coverage Δ
unit	57.6391% <100.0000%> (-0.0260%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

@@               Coverage Diff                @@
##             master     #11253        +/-   ##
================================================
- Coverage   57.6650%   57.6391%   -0.0260%     
================================================
  Files           850        850                
  Lines        126059     126218       +159     
================================================
+ Hits          72692      72751        +59     
- Misses        47971      48065        +94     
- Partials       5396       5402         +6     

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lidezhu, sdojjy

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [lidezhu,sdojjy]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-06-11 10:16:30.012940032 +0000 UTC m=+459744.066251955: ☑️ agreed by sdojjy.
• 2024-06-12 09:41:03.385866647 +0000 UTC m=+544017.439178571: ☑️ agreed by lidezhu.

[hongyunyan]

/retest

[hongyunyan]

/retest