ZigBee discovery updates

I’ve updated the ZigBee discovery so that it now discovers channels during the discovery process. This should significantly improve support for battery devices as it will allow the configuration of the device while it is awake - previously this was done when the thing is created, and for battery devices this doesn’t work well.

Unfortunately the OH concept for discovery is that only discovery should be done during discovery, and the initialisation should be done after the thing creation. There is therefore no easy way to pass the channel discovery information into the thing handler, so channel discovery is currently performed again when the thing handler initialises. This might fail, but the device will at least have been initialised earlier.

I’ll try and find some way to pass the information to improve this in future, but for now I’d welcome any comments (note this is only implemented in OH2.5 snapshot).