Snapshots with no record should maybe only show the attributes they know about

[runspired]

Basically, if a snapshot's internal-model does not have a record, then we know it is not the subject of a call to save and has not been used in the UI. This means that we're likely generating it for a findRecord request. We can optimize this situation by iterating attributes when generating the snapshot a bit differently:

data/addon/-private/system/snapshot.js

Lines 27 to 31 in 7b5a885

	// TODO is there a way we can assign known attributes without
	// using `eachAttribute`? This forces us to lookup the model-class
	// but for findRecord / findAll these are empty and doing so at
	// this point in time is unnecessary.
	internalModel.eachAttribute((keyName) => this._attributes[keyName] = internalModel.getAttributeValue(keyName));

cc @stefanpenner @hjdivad