Implement per-scope soft_max_size policy in vacuum_storage

#684 (closed) originally called for implementing the scope-level soft_max_size policy. We deferred this because it requires a trigger to maintain the total size consumed by all files in each scope for each store, and that turns out to be a bit involved: I think we have to update it on changes to (at least) any of FileInArtifact, FileInStore, and FileStoreInScope. It should still be doable, though, so I'm splitting out a separate issue for it.