routing+routerrpc: rename PaymentAttemptPenalty to AttemptCost

Make field names consistent with the command line flag.

routing/integrated_routing_context_test.go

@@ -65,8 +65,8 @@ func newIntegratedRoutingContext(t *testing.T) *integratedRoutingContext {
 		},
 
 		pathFindingCfg: PathFindingConfig{
-			PaymentAttemptPenalty: 1000,
-			MinProbability:        0.01,
+			AttemptCost:    1000,
+			MinProbability: 0.01,
 		},
 
 		source: source,