routing: use capacity in pathfinding

Extends the pathfinder with a capacity argument for later usage.

In tests, the inserted testCapacity has no effect, but will be used
later to estimate reduced probabilities from it.

routing/router.go

@@ -227,7 +227,7 @@ type MissionController interface {
 	// GetProbability is expected to return the success probability of a
 	// payment from fromNode along edge.
 	GetProbability(fromNode, toNode route.Vertex,
-		amt lnwire.MilliSatoshi) float64
+		amt lnwire.MilliSatoshi, capacity btcutil.Amount) float64
 }
 
 // FeeSchema is the set fee configuration for a Lightning Node on the network.