[WIP] online/offline syncing loses hasMany relationship

Related issue: https://github.com/kurko/ember-sync/issues/15#issuecomment-48251695.

Can't get to meaningful error messages yet, it stalls with:

> Error: Assertion Failed: You looked up the 'cartItems' relationship on '<(subclass of DS.Model):ember311:1>' but some of the associated records were not loaded. Either make sure they are all loaded together with the parent record, or specify that the relationship is async (`DS.hasMany({ async: true })`)
Same error message with fixture, and now earlier:

> Error: Assertion Failed: Error: Assertion Failed: You looked up the 'cartItems' relationship on '<(subclass of DS.Model):ember288:1>' but some of the associated records were not loaded. Either make sure they are all loaded together with the parent record, or specify that the relationship is async (`DS.hasMany({ async: true })`)