flusher: Add offset check before sync flusher

[taobun]

Implementation details

• Add offset check before handle message.
• Change a pruning strategy to syncable
• Remove unused script.

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• The pull request includes a description of the implementation/work done in detail.
• The pull request includes any and all appropriate unit/integration tests
• You have added a relevant changelog entry to CHANGELOG_UNRELEASED.md
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If this PR doesn't need to be merged into v1.0.6 please remove this option:

• The pull request has been cherry-picked to rc/1.0.6

[render]

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