proofs: change FetchProof in asset store to also look at outpoint

[guggero]

Background

Within our asset store database we currently assume script keys to be unique across assets in some cases.
But when receiving to the same TAP address multiple times, the AssetStore.FetchProof would currently fail because multiple items would be returned for the FetchAssetProof SQL query.

[jharveyb]

Resolved?

[guggero]

Fixed by #730.