- no more passing around "invoice" in GUIs, invoice "paid" detection is now handled by wallet logic
- a tx can now pay for multiple invoices
- an invoice can now be paid by multiple txs (through partial payments)
- new data structure in storage: prevouts_by_scripthash
- type: scripthash -> set of (outpoint, value)
- also, storage upgrade to build this for existing wallets
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