htlcswitch: modify the InvoiceDatabase interface to allow specifying final payment amt

In this commit, we modify the InvoiceDatabase slightly to allow the link
to record what the final payment about for an invoice was. It may be the
case that the invoice actually had no specified value, or that the payer
paid more than necessary. As a result, it's important that our on-disk
records properly reflect this.

To fix this issue, the SettleInvoice method now also accepts the final
amount paid.

Fixes #856.

htlcswitch/link.go

@@ -2298,8 +2298,9 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 			}
 
 			preimage := invoice.Terms.PaymentPreimage
-			err = l.channel.SettleHTLC(preimage,
-				pd.HtlcIndex, pd.SourceRef, nil, nil)
+			err = l.channel.SettleHTLC(
+				preimage, pd.HtlcIndex, pd.SourceRef, nil, nil,
+			)
 			if err != nil {
 				l.fail(LinkFailureError{code: ErrInternalError},
 					"unable to settle htlc: %v", err)
@@ -2307,8 +2308,11 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 			}
 
 			// Notify the invoiceRegistry of the invoices we just
-			// settled with this latest commitment update.
-			err = l.cfg.Registry.SettleInvoice(invoiceHash)
+			// settled (with the amount accepted at settle time)
+			// with this latest commitment update.
+			err = l.cfg.Registry.SettleInvoice(
+				invoiceHash, pd.Amount,
+			)
 			if err != nil {
 				l.fail(LinkFailureError{code: ErrInternalError},
 					"unable to settle invoice: %v", err)