Client::update_root should verify fetched metadata is expected version

[erickt]

When tuf::Client::update_root downloads root metadata from a server, it's currently not checking that that metadata's version matches the expected version. So this means if we are currently on version 5.root.json, when we try to download 6.root.json the server could actually give us version 7. We should make sure the metadata matches the expected version.

[erickt]

Here is the relevant section of the spec:

1.4. Check for a rollback attack. The version number of the trusted root metadata file (version N) must be less than or equal to the version number of the new root metadata file (version N+1). Effectively, this means checking that the version number signed in the new root metadata file is indeed N+1. If the version of the new root metadata file is less than the trusted metadata file, discard it, abort the update cycle, and report the rollback attack. On the next update cycle, begin at step 0 and version N of the root metadata file.