cached version numbers to check downloaded version numbers of cached data
validates against the old builder.
This also removes the `GetRepo` function of the builder and adds some data
accessors instead that are necessary to do a consistent download and check
versions, that way the downloader doesn't need to fish around in the repo
itself for data in order to figure out what to download.
Signed-off-by: Ying Li <ying.li@docker.com>
This means that the server cannot accept any roots not signed by an
x509 key whose GUN doesn't match the GUN being updated.
Signed-off-by: Ying Li <ying.li@docker.com>