Measuring topology performance. Fixing a defect that caused us to run w/o accumulator when config file was not present.

Change-Id: I5ad538b8a441cd6ff2aefea49a0def10b8e0f4d5
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/topology/impl/DefaultTopology.java b/core/store/dist/src/main/java/org/onlab/onos/store/topology/impl/DefaultTopology.java
index 4aafa35..939e558 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/topology/impl/DefaultTopology.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/topology/impl/DefaultTopology.java
@@ -65,6 +65,7 @@
             new TarjanGraphSearch<>();
 
     private final long time;
+    private final long computeCost;
     private final TopologyGraph graph;
 
     private final SCCResult<TopologyVertex, TopologyEdge> clusterResults;
@@ -104,6 +105,7 @@
 
         this.broadcastSets = buildBroadcastSets();
         this.infrastructurePoints = findInfrastructurePoints();
+        this.computeCost = Math.max(0, System.nanoTime() - time);
     }
 
     @Override
@@ -112,6 +114,11 @@
     }
 
     @Override
+    public long computeCost() {
+        return computeCost;
+    }
+
+    @Override
     public int clusterCount() {
         return clusters.size();
     }
@@ -453,6 +460,7 @@
     public String toString() {
         return toStringHelper(this)
                 .add("time", time)
+                .add("computeCost", computeCost)
                 .add("clusters", clusterCount())
                 .add("devices", deviceCount())
                 .add("links", linkCount())