[feature] Close previous sessions with same MAC address on new session

[nemesifier]

Whenever a new session is started, we shall send an update query which flags as closed any previous session from the same user and called-station-id which is still open, we can use "Session-Timeout" as termination cause and we can use the last update time as stop time.

This is needed because in some occasions users may be disconnected from the captive portal and will have to log in again, but the previous sessions will still be open, which can cause problems if the user has limits on the amount of time they can stay connected (because 2 sessions consume double the time and hence the available time finishes a lot faster).