sorter: fix memory leak in memoryBackEnd (#2665)

[ti-chi-bot]

This is an automated cherry-pick of #2665

What problem does this PR solve?

• When the sorter is using memory to sort, the sorter will internally hold a reference to events that have already been output to the sink. This will delay the recycling of the PolymorphicEvent objects by GC, causing increased memory use.

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

What is changed and how it works?

• memoryBackEnd now eagerly clears references to events not needed any more.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

Optimize memory management when unified sorter is using memory to sort.

[ti-chi-bot]

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 6a44714