`FilesystemStore::list` is not safe to call concurrently

[TheBlueMatt]

We can have a file/dir removed between fs::read_dir and dir_entry_is_key looks at its metadata which causes a spurious error. Obviously list will always return a set that may be bogus by the time its used, but that should be pretty clear from the API, the call shouldn't fail, however (and its probably okay that its internally-inconsistent).

[tnull]

Tbh. I'm a bit hesistant to break the approach of bubbling up any IO error to the callsite and have them deal with it based on the context. However, I agree that if any, list can be a bit more relaxed, now opened #3799.