multi: introduce new traffic shaper method.

We introduce a new specific fail resolution error when the
external HTLC interceptor denies the incoming HTLC. Moreover
we introduce a new traffic shaper method which moves the
implementation of asset HTLC to the external layers.
Moreover itests are adopted to reflect this new change.

invoices/invoiceregistry.go

@@ -1117,13 +1117,15 @@ func (i *InvoiceRegistry) notifyExitHopHtlcLocked(
 				return nil, nil
 			}
 
-			// If a cancel signal was set for the htlc set, we set
-			// the resolution as a failure with an underpayment
-			// indication. Something was wrong with this htlc, so
-			// we probably can't settle the invoice at all.
+			// The error `ExternalValidationFailed` error
+			// information will be packed in the
+			// `FailIncorrectDetails` msg when sending the msg to
+			// the peer. Error codes are defined by the BOLT 04
+			// specification. The error text can be arbitrary
+			// therefore we return a custom error msg.
 			resolution = NewFailResolution(
 				ctx.circuitKey, ctx.currentHeight,
-				ResultAmountTooLow,
+				ExternalValidationFailed,
 			)
 
 			// We cancel all HTLCs which are in the accepted state.

invoices/modification_interceptor.go

@@ -137,9 +137,6 @@ func (s *HtlcModificationInterceptor) Intercept(clientRequest HtlcModifyRequest,
 	// Wait for the client to respond or an error to occur.
 	select {
 	case response := <-responseChan:
-		log.Debugf("Received invoice HTLC interceptor response: %v",
-			response)
-
 		responseCallback(*response)
 
 		return nil

invoices/resolution_result.go

@@ -120,6 +120,10 @@ const (
 	// ResultAmpReconstruction is returned when the derived child
 	// hash/preimage pairs were invalid for at least one HTLC in the set.
 	ResultAmpReconstruction
+
+	// ExternalValidationFailed is returned when the external validation
+	// failed.
+	ExternalValidationFailed
 )
 
 // String returns a string representation of the result.
@@ -189,6 +193,9 @@ func (f FailResolutionResult) FailureString() string {
 	case ResultAmpReconstruction:
 		return "amp reconstruction failed"
 
+	case ExternalValidationFailed:
+		return "external validation failed"
+
 	default:
 		return "unknown failure resolution result"
 	}
@@ -202,7 +209,8 @@ func (f FailResolutionResult) IsSetFailure() bool {
 		ResultAmpReconstruction,
 		ResultHtlcSetTotalTooLow,
 		ResultHtlcSetTotalMismatch,
-		ResultHtlcSetOverpayment:
+		ResultHtlcSetOverpayment,
+		ExternalValidationFailed:
 
 		return true