Allow users to migrate b/w different KvStore implementations.

[G8XSU]

No description provided.

[tnull]

Note that we discussed this when introducing the KVStore trait upstream. I still think the if we want to support migration between stores it should only use trait methods to allow to write generalized migration logic.

I.e. we'd need to add a way to list primary and secondary namespaces to be able to walk the tree and migrate all keys of all namespaces.