sorter: reduce memory malloc to avoid too much CPU overhead (#1854)

[ti-chi-bot]

This is an automated cherry-pick of #1854

What problem does this PR solve?

ref: #1853

What is changed and how it works?

Since the access of h.heap is not concurrent, we can use the same slice

The heap will be used in an async pool. We choose a conservative initialize size for sort heap slice.

• the heap is flushed every 100ms or accumulated data size execeeds 1GB?

Check List

Tests

• Unit test
• Integration test

Release note

• Reduce memory malloc in sort heap to avoid too much CPU overhead.

[overvenus]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei
• overvenus

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 9a1a01a

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.0@ff9a93c). Click here to learn what that means.
The diff coverage is n/a.

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[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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/run-all-tests

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[amyangfei]

/run-kafka-tests