Improvements in group 0 behavior

[sidoh]

This change makes interplay between group 0 (the "all" group) and member groups more intuitive.

Previously, behavior was simple, but caused confusion when users expected group 0 to track its own state (e.g., #318). Since 1.8, group 0 state was not tracked (#284).

This change makes the interplay more complex, but hopefully more intuitive.

Summary

• Group 0 will have its own state. As was the case previously, applying a change to group 0 will fan out to individual groups.
• When an individual group changes a field, group 0 state for that field will be cleared.

Example

Group 0 is fanned out

$ curl -H 'content-type: application/json' -X PUT -d '{"status":"on","level":10,"kelvin":100}' milight-hub/gateways/0x2200/rgb_cct/0

$ curl milight-hub/gateways/0x2200/rgb_cct/0
{"status":"ON","level":10,"kelvin":100,"bulb_mode":"white","effect":"white_mode"}

$ curl milight-hub/gateways/0x2200/rgb_cct/1
{"status":"ON","level":10,"kelvin":100,"bulb_mode":"white","effect":"white_mode"}

$ curl milight-hub/gateways/0x2200/rgb_cct/2
{"status":"ON","level":10,"kelvin":100,"bulb_mode":"white","effect":"white_mode"}

Updating kelvin for Group 1 clears kelvin for Group 0

$ curl -H 'content-type: application/json' -X PUT -d '{"kelvin":10}' milight-hub/gateways/0x2200/rgb_cct/1

$ curl milight-hub/gateways/0x2200/rgb_cct/0
{"status":"ON","level":10,"bulb_mode":"white","effect":"white_mode"}

$ curl milight-hub/gateways/0x2200/rgb_cct/1
{"status":"ON","level":10,"kelvin":10,"bulb_mode":"white","effect":"white_mode"}

$ curl milight-hub/gateways/0x2200/rgb_cct/2
{"status":"ON","level":10,"kelvin":100,"bulb_mode":"white","effect":"white_mode"}