Generate CA Certificates that are valid for a bit further back in time.

See bug #1352944. We were generating certificates that were valid 5 minutes ago, to avoid
problems with the clock on the client being out of sync with the clock on the server, but
it seems 5 minutes isn't quite enough to account for real world clock skew. So bump it
up to 1 week.
add some comments about what we are testing.

Also, do the same work for Server and Client certs as well as the CA cert.
CheckSum -> Checksum
Add some explicit backups metadata doc validation.
Move to using filestorage.FileMetadata and read-only fields for backups metadata.
Add a storage abstraction for backup archives.
Move backups metadata into own package.
backup -> backups.
Add documentation for NewBackupsOrigin.
Accommodate a default version.
Clean up the doc field checking.
Correct the package referenced relative to Metadata.
Address use of panic.
Add a note about why Origin is a separate type from Metadata.
Fix an error message.
Use a more canonical form for testing interface satisfaction.
Explain why there are unimplemented methods in backupMetadataStorage.
Drop the superfluous "Backups" from test names.
Remove divider comments.
Switch to the real canonical form.