Track inbound payments by also unique payment IDs

[tnull]

After #256, we won't track inbound BOLT12 payments until PaymentClaimable, which differs from the prior BOLT11 behavior. Eventually we should:

• Add an invoice/offer store (see Make invoice storage optional #51) and register created invoices in this rather than the payment store. The invoice/offer store entries should have a link to the associated PaymentId(s)
• Add a PaymentId field to PaymentClaimable/PaymentReceived, e.g., derived from the HTLC Id + inbound channel
• Then migrate to track all payments (inbound or outbound) only by the LDK-provided PaymentIds.