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/router_test.go

@@ -2960,6 +2960,7 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 			ChainView:          chainView,
 			Control:            control,
 			SessionSource:      &mockPaymentSessionSource{},
+			MissionControl:     &mockMissionControl{},
 			Payer:              payer,
 			ChannelPruneExpiry: time.Hour * 24,
 			GraphPruneInterval: time.Hour * 2,