core: use markers in StatusDataset and NotificationStream

This commit also refactors FaceMonitor as generic NotificationSubscriber,
and refactors AutoregServer to use FaceMonitor.

refs #1837 #1838

Change-Id: I8b40dfae118853d1224c8290cf92e7cc0daa116f
diff --git a/rib/rib-manager.cpp b/rib/rib-manager.cpp
index 318198a..d4be75d 100644
--- a/rib/rib-manager.cpp
+++ b/rib/rib-manager.cpp
@@ -118,8 +118,8 @@
   }
 
   NFD_LOG_INFO("Start monitoring face create/destroy events");
-  m_faceMonitor.addSubscriber(bind(&RibManager::onNotification, this, _1));
-  m_faceMonitor.startNotifications();
+  m_faceMonitor.onNotification += bind(&RibManager::onNotification, this, _1);
+  m_faceMonitor.start();
 
   fetchActiveFaces();
 }