implement station inactivity timeout

After the max station inactivity has expired, a probe request
(a data null frame) is sent to the station. It the station does
not ACK this frame, it is removed from the WTP.

Note: inactivity timeout is not the same as the CAPWAP Idle Timeout.
The CAPWAP Idle Timeout would remove a station due to inactivity
even when it is still reachable from the WTP. In contrast, the
inactivity timeout probes whether the station is still present
and only removes it when not.

conf/wtp.conf

@@ -49,6 +49,7 @@ application: {
 
 	timer: {
 		statistics = 120;
+		inactivity = 300;
 	};
 
 	dtls: {