Connection check in macpassd + ip changes

[samuelemusiani]

Check periodically if a device is stil connected. If not, after a short period of time, drop the connection

[samuelemusiani]

Partially done in c00cc5c

[samuelemusiani]

We need to make sure that if a device is not connected, another device that take his ip does not responds for him. If two devices have the same ip?

[samuelemusiani]

Partially done in 164d349. I do not close this yet as some test need to be done