proposal: add lastupdated field to Device models/devices/device.go

[jonaz]

What do you think @stamp ? Currently some devices like xioami zigbee devices have it in their state. Perhaps would be nice to always put it in the device at the time the server gets the update-devices message. That way the server could also have a timeout period when it sets the devices as Online=false.

edit: the timeout thing might only be applicable to sensors.

[stamp]

I'm thinking a solution that works something like this

• [ ] Add a lastSeen
• [ ] Add a label or flag that identifies a device as battery operated
• [ ] Automatically generate and maintain notification rules based on lastSeen for each battery operated sensor