client monitor

include/emqttd.hrl

@@ -91,6 +91,7 @@
 -record(mqtt_client, {
     client_id     :: binary() | undefined,
     client_pid    :: pid(),
+    client_mon    :: reference(),
     username      :: binary() | undefined,
     peername      :: {inet:ip_address(), integer()},
     clean_sess    :: boolean(),