sink, sorter: do not hog memory (#2606)

[ti-chi-bot]

This is an automated cherry-pick of #2606

What problem does this PR solve?

Reduce memory usage for sink node and file backend.

Cc https://github.com/pingcap/ticdc/issues/2553

Check List

Tests

• Manual test (add detailed scripts or steps below)

Related changes

• Need to cherry-pick to the release branch

Release note

None

[ti-chi-bot]

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[overvenus]

/merge

[ti-chi-bot]

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

Commit hash: 07178fc

[overvenus]

/merge

[amyangfei]

/run-integration-tests
/run-kafka-tests

[amyangfei]

/run-integration-tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.0@0999a92). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             release-5.0      #2644   +/-   ##
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  Hits                   ?      10915           
  Misses                 ?       7751           
  Partials               ?       1043           

[ti-chi-bot]

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