Update isReloading when a record is reloaded in the background

[felixkiss]

As discussed with @bmac on Slack:

Currently, neither record.isLoading nor record.isReloading is set to true while the adapter performs a background reload of the specific record.

At least isReloading should be true during the time the adapter waits for a server response.

[pete-the-pete]

@runspired after a quick and random look through the code, it seems that reload has some pretty specific meaning...the app is explicitly calling reload on the model or relationship. Does it make sense to have a new property to signify backgroundReloading specifically?

Also, the comments at data/addon/-private/system/model/states.js are pretty helpful, sad I missed the before

[runspired]

We believe the request state RFC provides infrastructure necessary for an add-on or app to expose a flag for whether a record has any pending requests (not just a primary reload or a background reload).

[ghost]

This is the RFC: https://emberjs.github.io/rfcs/0466-request-state-service.html?highlight=request#request-state-service