Fix batch model loading to prevent triggering multiple events

Previously, whenever loading a set of objects, it would run `Model._mapIdentity` for each element loaded. This would trigger a ton of events, and end up creating a lot of extra nodes that would be subsequently removed.

This change properly batches loading so that only a single event is fired.

Paired with @SoapyIllusions 

@nickjs @hornairs @kristianpd for review