Replicate versions from one archive to another

[sourcefrog]

It would be useful to have an archive on local, limited-size, fast storage, and then to periodically copy some versions to a second archive that is perhaps bigger but intermittently connected or slower.

Probably it's less confusing if the versions have to stay in sync and so the band ids to be copied must not be present in the destination.

This might be as simple as:

• [ ] Select versions to copy. By default, everything not present in the destination, but probably there should be a way to be more specific.
• [ ] For each version, copy all the referenced blocks (if not already present) and then copy all the index hunks.