cnct+htlcswitch+invoices: report circuit key to invoice registry

Currently the invoice registry cannot tell apart the htlcs that pay to
an invoice. Because htlcs may also be replayed on startup, it isn't
possible to determine the total amount paid to an invoice.

This commit is a first step towards fixing that. It reports the circuit
keys of htlcs to the invoice registry, which forms the basis for
accurate invoice accounting.

htlcswitch/link.go

@@ -2878,9 +2878,14 @@ func (l *channelLink) processExitHop(pd *lnwallet.PaymentDescriptor,
 	// receive back a resolution event.
 	invoiceHash := lntypes.Hash(pd.RHash)
 
+	circuitKey := channeldb.CircuitKey{
+		ChanID: l.ShortChanID(),
+		HtlcID: pd.HtlcIndex,
+	}
+
 	event, err := l.cfg.Registry.NotifyExitHopHtlc(
 		invoiceHash, pd.Amount, pd.Timeout, int32(heightNow),
-		l.hodlQueue.ChanIn(), eob,
+		circuitKey, l.hodlQueue.ChanIn(), eob,
 	)
 
 	switch err {