routing: global probability based mission control

Previously every payment had its own local mission control state which
was in effect only for that payment. In this commit most of the local
state is removed and payments all tap into the global mission control
probability estimator.

Furthermore the decay time of pruned edges and nodes is extended, so
that observations about the network can better benefit future payment
processes.

Last, the probability function is transformed from a binary output to a
gradual curve, allowing for a better trade off between candidate routes.

routing/pathfind.go

@@ -242,7 +242,8 @@ type graphParams struct {
 type RestrictParams struct {
 	// ProbabilitySource is a callback that is expected to return the
 	// success probability of traversing the channel from the node.
-	ProbabilitySource func(route.Vertex, EdgeLocator) float64
+	ProbabilitySource func(route.Vertex, EdgeLocator,
+		lnwire.MilliSatoshi) float64
 
 	// FeeLimit is a maximum fee amount allowed to be used on the path from
 	// the source to the target.
@@ -398,7 +399,7 @@ func findPath(g *graphParams, r *RestrictParams, source, target route.Vertex,
 		// Request the success probability for this edge.
 		locator := newEdgeLocator(edge)
 		edgeProbability := r.ProbabilitySource(
-			fromVertex, *locator,
+			fromVertex, *locator, amountToSend,
 		)
 
 		log.Tracef("path finding probability: fromnode=%v, chanid=%v, "+