Should we stop directly storing metadata MetadataVersion::None?

[erickt]

As part #151, I started making rust-tuf concurrent. This means however that some previously safe patterns, like storing metadata both as MetadataVersion::Number(X) and MetadataVersion::None to signify it as the latest version is now racy since other threads can compete on updating metadata. What do you think about getting rid of MetadataVersion::None, and changing Repository::fetch_metadata to take a Option<MetadataVersion>, where None signifies getting the newest version?