incrementalFetch(): fix race condition which would miss rows based on changed_at #288
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incrementalFetch() relies on every newly inserted row having a higher changed_at than every already committed one. At the moment, web doesn't guarantee this. It even writes changed_at rounded to whole seconds. Hence, we could skip one of two rows which appeared during the same second.
This change simply ignores the most recent 3s of config updates (for 3s), so that two or more new rows which appear during the same second aren't a problem. They're processed together 3s after they get committed.