routing: process successes in mission control

This commit modifies paymentLifecycle so that it not only feeds
failures into mission control, but successes as well.
This allows for more accurate probability estimates. Previously,
the success probability for a successful pair and a pair with
no history was equal. There was no force that pushed towards
previously successful routes.

routing/pathfind.go

@@ -55,7 +55,7 @@ var (
 
 	// DefaultAprioriHopProbability is the default a priori probability for
 	// a hop.
-	DefaultAprioriHopProbability = float64(0.95)
+	DefaultAprioriHopProbability = float64(0.6)
 )
 
 // edgePolicyWithSource is a helper struct to keep track of the source node