[Link Event Damping] Add per port link event damper class.

- Adding per port link event damper class that manages the internal
  behavior and workings of link event damping logic on a port where link
  event damping config is enabled.
- This class keeps track of damping timer, current port state, post
  damping advertised port state and relevant debug counters per port.

HLD: sonic-net/SONiC#1071

syncd/Makefile.am

@@ -32,6 +32,7 @@ libSyncd_a_SOURCES = \
 				NotificationHandler.cpp \
 				NotificationProcessor.cpp \
 				NotificationQueue.cpp \
+				PortLinkEventDamper.cpp \
 				PortMap.cpp \
 				PortMapParser.cpp \
 				PortStateChangeHandler.cpp \

syncd/NotificationHandler.cpp

@@ -132,6 +132,17 @@ void NotificationHandler::updateNotificationsPointers(
     }
 }
 
+void NotificationHandler::onPortStateChangePostLinkEventDamping(
+        _In_ uint32_t count,
+        _In_ const sai_port_oper_status_notification_t *data)
+{
+    SWSS_LOG_ENTER();
+
+    auto s = sai_serialize_port_oper_status_ntf(count, data);
+
+    enqueueNotification(SAI_SWITCH_NOTIFICATION_NAME_PORT_STATE_CHANGE, s);
+}
+
 // TODO use same Notification class from sairedis lib
 // then this will handle deserialize free
 

syncd/NotificationHandler.h

@@ -35,6 +35,12 @@ namespace syncd
                     _In_ uint32_t attr_count,
                     _In_ sai_attribute_t *attr_list) const;
 
+            // Handling of port state change event when received after being
+            // processed in link event damping logic.
+            virtual void onPortStateChangePostLinkEventDamping(
+                    _In_ uint32_t count,
+                    _In_ const sai_port_oper_status_notification_t *data);
+
         public: // members reflecting SAI callbacks
 
             void onFdbEvent(