Tighten/simplify implementation of HoistModel.componentProps

[lbwexler]

• Fix difficult bug with re-render loops.
• Avoid mobx object/proxy object creation for models that don't use this property
• Avoid extra prop processing for linked models that don't use this property

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• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes, added breaking-change label + CHANGELOG if so.
• Updated doc comments / prop-types, or determined not required.
• Reviewed and tested on Mobile, or determined not required.
• Created Toolbox branch / PR, or determined not required.

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[ghsolomon]

Nice cleanup!

[amcclain]

I am sure this is a great cleanup, but admit I don't really understand it.

Were we previously doing the apply and delete checks on the singular _componentProps object to trigger reactions to the proxied observable?

To put it another way, we had/still have the comment on the getter:

Observability is based on a shallow computation for each prop (i.e. a reference change in any particular prop will trigger observers to be notified).

Is it correct to say that the @computed({equals: comparer.shallow}) annotation is now making that the case? Was that the "extra processing" referenced by:

Avoid extra prop processing for linked models that don't use this property