routing: remove sourceNode from routingGraph interface

In this commit, we further reduce the routingGraph interface and this
time we make it more node-agnostic so that it can be backed by any graph
and not one with a concept of "sourceNode".

routing/payment_session_source.go

@@ -73,8 +73,8 @@ func (m *SessionSource) NewPaymentSession(p *LightningPayment) (
 	}
 
 	session, err := newPaymentSession(
-		p, getBandwidthHints, m.getRoutingGraph,
-		m.MissionControl, m.PathFindingConfig,
+		p, m.SourceNode.PubKeyBytes, getBandwidthHints,
+		m.getRoutingGraph, m.MissionControl, m.PathFindingConfig,
 	)
 	if err != nil {
 		return nil, err