Redis migration

This is the code for the redis migration backend.

It takes an old and a new backend, and tries to copy existing keys to the new database before answering anything.
Yes, but that's less likely: e.g. if would need someone to copy over prod settings to a test.json file etc. Of course there are ways to do this, there are always ways. I don't want to prevent them all, but just the one that seems more likely (to me, at least).

If you fill that doesn't matter then we don't agree, but having this protection in place is just a "garde fou".