Avoid memory leaks by holding the .model and .modelDelegate properties in the JS wrapper

[ajklein]

Right now, .model and .modelDelegate are in NodeRareData as strong references (in a ScriptValue). This will cause memory leaks if the model or modelDelegate have a reference back to the DOM. The easiest solution will be to move them to the JS wrapper, and store them in either internal fields or (more likely) hidden properties.