* Cache the name of LIFX lights
After #7031 the LIFX device will change during an unregister/register
transition. This has the user-visible effect of the new device missing
a friendly name until the next poll.
We now cache the name internally and it will then transfer to the new
device when it registers.
* Allow LIFX logging even without an available device
This will allow us to set the device to None when it unregisters.
* Calculate LIFX availability from the existence of a device
This has become possible because the device is no longer needed
to provide the name of the light when it is unavailable.
We just have to forget the device when it unregisters.