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/mock_test.go

@@ -105,7 +105,7 @@ func (m *mockMissionControl) ReportPaymentFail(paymentID uint64,
 	return false, 0, nil
 }
 
-func (m *mockMissionControl) GetEdgeProbability(fromNode route.Vertex, edge EdgeLocator,
+func (m *mockMissionControl) GetProbability(fromNode, toNode route.Vertex,
 	amt lnwire.MilliSatoshi) float64 {
 
 	return 0
@@ -130,12 +130,6 @@ func (m *mockPaymentSession) RequestRoute(payment *LightningPayment,
 	return r, nil
 }
 
-func (m *mockPaymentSession) ReportVertexFailure(v route.Vertex) {}
-
-func (m *mockPaymentSession) ReportEdgeFailure(failedEdge edge, minPenalizeAmt lnwire.MilliSatoshi) {}
-
-func (m *mockPaymentSession) ReportEdgePolicyFailure(failedEdge edge) {}
-
 type mockPayer struct {
 	sendResult       chan error
 	paymentResultErr chan error