Record the time of the last full update in interface structure.

interface.h

@@ -86,6 +86,7 @@ struct interface {
     time_t bucket_time;
     unsigned int bucket;
     time_t activity_time;
+    time_t last_update_time;
     unsigned short hello_seqno;
     unsigned hello_interval;
     unsigned update_interval;

message.c

@@ -1132,6 +1132,7 @@ send_update(struct interface *ifp, int urgent,
             }
         }
         set_timeout(&ifp->update_timeout, ifp->update_interval);
+        ifp->last_update_time = now.tv_sec;
     }
     schedule_update_flush(ifp, urgent);
 }