Major Description
We need a way to backup and restore a repository. I was thinking that the MK impl could expose an interface for this, as the actual implementation would differ quite a bit between e.g. TarMK and MongoMK.
Also, I think we could leverage the MVCC nature of the MKs and mark a specific revision as "the revision to backup" (regardless of ongoing writes). That would allow us to prevent the ugly situation in JR2, that we need to stop writes for a while to produce a consistent backup.
The restore in such a scenario would discard revisions that happened after said marker (but still made it into the backup).