Improve rebuild logic

[LeeSmet]

The current rebuild logic is fairly simple: retrieve data, reencode, and send back to the new backends. However, we can check if any of the new backends is also used in the old metadata. If it is, we can assign it the same shard, eliminating the write to that backend, saving some space.

Need toch check if encoding is deterministic for this, especially if it is a parity shard