Looks like I broke this in a refactoring, probably
adc6c404. Snapshots loaded for the overview
are almost always available in cache before a webpage is visited, so
those pages would never get updated. We need to update stale snapshots
in ephy_snapshot_service_get_snapshot_path_async() to avoid this. It
*could* still happen that snapshot requests are scheduled multiple
times in a row, but it's unlikely and harmless.