Unloading models can leave stale fragments w/ ember-data-2.13

[workmanw]

Steps:

1. pushPayload that includes a model with embedded fragment
2. find the record and get a property on the embedded fragment.
3. unload model
4. pushPayload again for the same model
5. find the same record again and try to use the embedded fragment. You'll find it's destroyed and no longer observable.

I bisected the issue and found this beastly commit is the culprit: emberjs/data@2f18405 ... I really have no idea where to begin 😢

[workmanw]

Here is the PR for it: emberjs/data#4593

[workmanw]

Okay, I've fully grokked the PR and resurrected all the ember-data internals knowledge from the cold storage part of my brain 😜 . This seems to be related to relationships. It seems that internalModels w/ a relationship are preserved when unloadRecord is called. So we need to do a better job at cleaning up.

Let me see if I can write a failing test now that I understand what's happening.