[Meta Issue] remove instance state from ModelFactories

[stefanpenner]

specifically Model.store and anything else that pollutes.

this will let us fix:

• typeKey
• aspects of polymorphic
• likely more.

[sly7-7]

@stefanpenner I tried to investigate where the store was needed in the factories, and I don't know what strategy we could use to remove it from the relationships descriptors like in https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/relationships/ext.js#L55, where the store is used to retrieve the type from relationship's meta.
I wish I could help on this one, but I need some insight to know where/how to start 😞

[igorT]

This is a dupe of a similar tracking bug, and is currently being handled by #3074