routing+routerrpc: expose mission control parameters in lnd config

This commit exposes the three main parameters that influence mission
control and path finding to the user as command line or config file
flags. It allows for fine-tuning for optimal results.

routing/payment_session_test.go

@@ -35,6 +35,7 @@ func TestRequestRoute(t *testing.T) {
 	session := &paymentSession{
 		mc: &MissionControl{
 			selfNode: &channeldb.LightningNode{},
+			cfg:      &MissionControlConfig{},
 		},
 		pathFinder: findPath,
 	}