fix(batch): unpin snapshot until the end of distributed batch query

[hzxa21]

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What's changed and what's your intention?

As titled. Fix the potential race between FE and CN on pinned epoch. This fix is valid for version <= 2.0 because in 2.1 we already deprecate safe epoch completely.

See https://github.com/risingwavelabs/risingwave/issues/17965#issuecomment-2274915561

Checklist

• [ ] I have written necessary rustdoc comments
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Release note

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[graphite-app[bot]]

Graphite Automations

"release branch request review" took an action on this PR • (11/18/24)

1 reviewer was added to this PR based on xxchan's automation.