Make UtxoSet store (height, txo) pairs rather than just the raw txo

Will take some experimentation to see if this is what I want the API
to be, if the memory usage is acceptable, etc.

This will force a total reindex for wizards-wallet users.

[breaking-change]
Add a `clone` to script sat-checker to satisfy new soundness rules

This was actually a soundness bug waiting to happen fwiw...but I'm
still not so happy about it.