ONOS-1440: Implements port statistics feature, which polls port statistics of all devices every 10 seconds. Also, implemented a simple portstats ONOS CLI command to show the statistics.

Change-Id: I57e046ae2c2463a58b478d3a5b523422cde71ba2
diff --git a/core/api/src/test/java/org/onosproject/net/device/DeviceServiceAdapter.java b/core/api/src/test/java/org/onosproject/net/device/DeviceServiceAdapter.java
index 422d062..4f72702 100644
--- a/core/api/src/test/java/org/onosproject/net/device/DeviceServiceAdapter.java
+++ b/core/api/src/test/java/org/onosproject/net/device/DeviceServiceAdapter.java
@@ -69,6 +69,11 @@
     }
 
     @Override
+    public List<PortStatistics> getPortStatistics(DeviceId deviceId) {
+        return null;
+    }
+
+    @Override
     public Port getPort(DeviceId deviceId, PortNumber portNumber) {
         return null;
     }