routing: use probability source in path finding

This PR replaces the previously used edge and node ignore lists in path
finding by a probability based system. It modifies path finding so that
it not only compares routes on fee and time lock, but also takes route
success probability into account.

Allowing routes to be compared based on success probability is achieved
by introducing a 'virtual' cost of a payment attempt and using that to
translate probability into another cost factor.

routing/payment_session.go

@@ -143,6 +143,20 @@ func (p *paymentSession) ReportEdgePolicyFailure(
 	p.errFailedPolicyChans[*failedEdge] = struct{}{}
 }
 
+func (p *paymentSession) getEdgeProbability(node route.Vertex,
+	edge EdgeLocator) float64 {
+
+	if _, ok := p.pruneViewSnapshot.vertexes[node]; ok {
+		return 0
+	}
+
+	if _, ok := p.pruneViewSnapshot.edges[edge]; ok {
+		return 0
+	}
+
+	return 1
+}
+
 // RequestRoute returns a route which is likely to be capable for successfully
 // routing the specified HTLC payment to the target node. Initially the first
 // set of paths returned from this method may encounter routing failure along
@@ -200,11 +214,11 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 			bandwidthHints:  p.bandwidthHints,
 		},
 		&RestrictParams{
-			IgnoredNodes:      pruneView.vertexes,
-			IgnoredEdges:      pruneView.edges,
-			FeeLimit:          payment.FeeLimit,
-			OutgoingChannelID: payment.OutgoingChannelID,
-			CltvLimit:         cltvLimit,
+			ProbabilitySource:     p.getEdgeProbability,
+			FeeLimit:              payment.FeeLimit,
+			OutgoingChannelID:     payment.OutgoingChannelID,
+			CltvLimit:             cltvLimit,
+			PaymentAttemptPenalty: DefaultPaymentAttemptPenalty,
 		},
 		p.mc.selfNode.PubKeyBytes, payment.Target,
 		payment.Amount,