sorter: fix unified sorter magic numbers & reduce memory consumption by channels (#1915)

[ti-chi-bot]

This is an automated cherry-pick of #1915

What problem does this PR solve?

• Unexpectedly large memory consumption by Unified Sorter when tables are many.
• Too many magic numbers in Unified Sorter.

What is changed and how it works?

• Reduce heapSorter inputCh size
• Add constants for some magic numbers used for sizes to better manage these sizes.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix Unified Sorter memory consumption when tables are many.

[liuzix]

/run-unit-tests

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei
• liuzix

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[lonng]

/merge

[ti-chi-bot]

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

Commit hash: 5cfbe19

[amyangfei]

/run-kafka-tests

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

Merging #1957 (76c9a8a) into release-4.0 (753073a) will decrease coverage by 0.4012%.
The diff coverage is 62.5927%.

@@                 Coverage Diff                 @@
##           release-4.0      #1957        +/-   ##
===================================================
- Coverage      53.7661%   53.3649%   -0.4013%     
===================================================
  Files              153        163        +10     
  Lines            15958      17296      +1338     
===================================================
+ Hits              8580       9230       +650     
- Misses            6475       7114       +639     
- Partials           903        952        +49