routing: convert to node pair based

Previously mission control tracked failures on a per node, per channel basis.
This commit changes this to tracking on the level of directed node pairs. The goal
of moving to this coarser-grained level is to reduce the number of required
payment attempts without compromising payment reliability.

routing/payment_session.go

@@ -91,7 +91,7 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	ss := p.sessionSource
 
 	restrictions := &RestrictParams{
-		ProbabilitySource: ss.MissionControl.GetEdgeProbability,
+		ProbabilitySource: ss.MissionControl.GetProbability,
 		FeeLimit:          payment.FeeLimit,
 		OutgoingChannelID: payment.OutgoingChannelID,
 		CltvLimit:         cltvLimit,