Handle a use case with frequent in and out sequences

[samip5]

Use case is when a user goes outside the Wifi range, and back multiple times a day, it shouldn't notifiy every time.
This should be enabled only explicly per user.

[zouppen]

There is a "flappiness" feature which requires given number of DHCP renewals to mark the person as arrived, but no "leave-flappiness" which would keep the person there for a given period of time. Flappiness helps in case user just walks by the premises and is not staying.