Support Server Polling

Hi!

I ran into a situation where I want my page to be infinitely scrollable but also populate the top of the page with new objects that come in from the server.

So right now I have an infinity model that successfully loads data, but I have a polling event that refreshes some of the loaded data, which breaks data relationships in the infinity model.

I was able to fix it for my use case, but it would be ideal to incorporate an update option that doesn't break relationships of non-updated objects.

It would be great if I could ask the infinity model to basically make the first call again and have it update the store with new objects, or any objects that have been updated.

Here's the polling code (which is managed by ember-concurrency):
```
this.get('item').reload().then(item => {
item.get('allOrders').reload()
});
```

And the infinity model is loaded through a model hook on a route.

I should also note that I'm using cursor based pagination, and the results coming back from the server are already ordered.