Move to Immutable Arrays

A concern I have is that Ember Infinity heavily relies on "pushing objects" into arrays. I don't think this is a great approach - as it relies on Ember's pushObjects API staying consistent - the `_internalModel` stuff is confusing for folks.

Instead I think we should be creating new arrays based on the existing & new data.

@davidgoli also raised some concern about using the store's `query` object in ember infinity. I agree. David - can you share some of your thoughts here?