Relinquish mastership, tests, and few modifications to trivial MastershipStore

Change-Id: Iae29de010f13cb3ee02bcb316510cc254d5756fc
diff --git a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
index eeab313..f8f4750 100644
--- a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
@@ -204,8 +204,11 @@
             checkNotNull(deviceId, DEVICE_ID_NULL);
             checkValidity();
             DeviceEvent event = store.markOffline(deviceId);
+
+            //we're no longer capable of mastership.
             if (event != null) {
                 log.info("Device {} disconnected", deviceId);
+                mastershipService.relinquishMastership(deviceId);
                 post(event);
             }
         }