processor,sink(cdc): let sink report resolved ts and do not skip buffer sink flush (#3540)

[ti-chi-bot]

This is an automated cherry-pick of #3540

What problem does this PR solve?

Revert #3029 and #2617 temporary for resolving #3503.

Cc https://github.com/pingcap/ticdc/issues/3545

Check List

Tests

• Integration test

See #3503

Release note

None

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[VelocityLight]

/merge

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[overvenus]

/merge

[ti-chi-bot]

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

Commit hash: a251cea

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[codecov-commenter]

Codecov Report

Merging #3561 (4016739) into release-4.0 (753073a) will increase coverage by 3.3475%.
The diff coverage is 67.1586%.

@@                 Coverage Diff                 @@
##           release-4.0      #3561        +/-   ##
===================================================
+ Coverage      53.7661%   57.1136%   +3.3475%     
===================================================
  Files              153        165        +12     
  Lines            15958      19582      +3624     
===================================================
+ Hits              8580      11184      +2604     
- Misses            6475       7338       +863     
- Partials           903       1060       +157