flush cache before segment merge (#4955)

[ti-chi-bot]

This is an automated cherry-pick of #4955

What problem does this PR solve?

Issue Number: close #4956

Problem Summary:
When do segment split, we try to copy the tail column files in the delta layer of the original segment to the new result
segments. So the new segments may contain data that doesn't belong to its segment range.

And this is ok for most cases, because the redundant data will be filtered out by the segment range when serve the read requests to the segment. So the redundant is invisible in almost all cases.

But when do segment merge later, if the previous redundant data is still not flushed to disk, it will be directly copied to the new merged segment again.

So the redundant data in each segment become visible again after segment merge which may cause potential data incorrectness.

What is changed and how it works?

Flush cache before every merge operation. So the potential unsaved data will be filtered out by the segment range when do merge.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Fix potential wrong result after a lot of insert and delete operations

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[JaySon-Huang]

close as no further patch version for the 6.0 branch