Draft: drivers/buffer-light: Use async IO to be notified about sensor changes

Instead of doing a non-blocking read of the sensor using a fixed timeout, we can also use the GLib async IO functions and wake up the mainloop every time a new sensor value becomes available.

Note that the old approach had a somewhat implicit rate limiting of sensor updates to 700 ms, which this approach no longer has. If we want such a rate limiting, that should probably be applied on a global level in iio-sensor-proxy though, instead of the per-driver level.