Use a native class instead of extending EmberObject for M3 Model

[runspired]

With the custom record class work landing we will soon no longer require using Ember's factory system. Currently using a little bit of "intimate knowledge of how things work" we can already eliminate use of EmberObject as the base class but with the landing of the RFC this becomes even more explicitly supported.

Changing the base class would improve creation performance and reduce the surface area of inherited APIs. We can still very easily provide some APIs such as get/set/setProperties for convenience.