Design and implement a proper backup and restore API

The current backup and restore API in org.apache.jackrabbit.oak.backup refers to classes and interfaces that should remain private to the oak-segment-tar bundle. This, in fact, is the reason why that package was not exported as part of the effort for OAK-4843. The current backup and restore API should be redesigned to be used from the outside of oak-segment-tar without exporting implementation details.