chore(state): add some more details around state management

[Tieske]

especially around safely adding/removing child relationships

[schaze]

I do not like the interweaved coupling of parent and child updates. This will make an implementation very cumbersome. Also you cannot guarantee another mqtt client will receive updates for different topics in sequential manor. In addition I want to keep coupling between parent and child as minimal as possible. Exact sequence and timing in MQTT do not work well in my experience.

I would clearly split this up and also switch the order when adding a child to first make sure the devices are published (with at least $state:init) before adding them to the parent (this way one could add multiple child devices with a single parent update).

Adding:

• Publish 1..n child(-ren) (as defined in spec - $state:init, $description, ..., ..., $state:ready)
• update parent's children property (as defined in the spec, $state:init, $description, state:ready)

Removing, whilst leaving the device on the Homie network:

• update parent's children property [remove 1..n children] (as defined in the spec, $state:init, $description, $state:ready)
• update child-device's description with cleared "root" and "parent" attributes (as defined in spec, $state: init, $description)
• set child's state to disconnected

Removing, permanently

• update parent's children property [remove 1..n children] (as defined in the spec, $state:init, $description, $state:ready)
• set child's $state to init
• clear child-device description (clear the $description attribute topic)
• clear all child-device topics (order is irrelevant as without a $description there is no device)

[Tieske]

@schaze thx, make sense. Updated.

I removed the "Removing, whilst leaving the device on the Homie network" part, since that is something that works in theory, but really do not see a real use-case for it.