*: reduce memory consumption by reducing channel sizes (#1999)

[ti-chi-bot]

This is an automated cherry-pick of #1999

What problem does this PR solve?

• TiCDC can OOM frequently on machines with memory less than 16GB.

What is changed and how it works?

• Decreased the channel sizes in two places.

Check List

Tests

• Unit test
• Integration test
• Manual test

Side effects

• Possible performance regression

Related changes

• Need to cherry-pick to the release branch

Release note

• Reduce unnecessary memory consumption

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 16b8968

[codecov-commenter]

Codecov Report

Merging #2011 (9dcdaf1) into release-4.0 (753073a) will decrease coverage by 1.1820%.
The diff coverage is 58.8892%.

@@                 Coverage Diff                 @@
##           release-4.0      #2011        +/-   ##
===================================================
- Coverage      53.7661%   52.5840%   -1.1821%     
===================================================
  Files              153        165        +12     
  Lines            15958      17511      +1553     
===================================================
+ Hits              8580       9208       +628     
- Misses            6475       7334       +859     
- Partials           903        969        +66