[bugfix] Fix temp table deletion causing runaway allocations

[tsmethurst]

Description

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This pull request addresses an issue we've been having with runaway memory allocations causing stress on the garbage collector. For some reason, deleting temporary tables causes SQLite to flip out and keep allocating memory, which is then immediately cleaned up.

This PR fixes the issue by changing the way we delete temporary tables: for Postgres, we can just use ON COMMIT DROP, which causes the temp tables to be cleaned up when the transaction ends. For SQLite, we can adjust the max conn lifetime to 5 minutes to ensure that temp tables get closed when the connection is closed + recreated.

Checklist

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