Parcel delivery confirmation handles all asset transfers associated with anchor point

[ffranr]

AssetStore.ConfirmParcelDelivery is a function which confirms package delivery as a final step in the asset send process. Confirming delivery entails reading asset transfer/delta objects from the db and then writing updated witness and proof objects to the db.

Currently, for a given on-chain anchor point, this function only handles one asset transfer. This function should be modified such that it can handle all asset transfers associated with a given anchor point.

This change will allow the reanchoring of passive asset within the same atomic db transaction that we use to confirm package delivery.

[guggero]

I think this is no longer relevant after the latest series of refactor, @ffranr?