Do not cache model factories on meta, or on other model CPs

Relationships are hard. Ember-Data has been storing data about a
relationship on the meta value for that CP. CPs hang around though, and
as a result that data on the meta becomes a cache. For some details
about a relationship, this is fine.

If User has many Posts, and you use User via a container, it would cache
a reference to the Post factory. But if you then destroy that container,
the reference to Post still exists in the CP meta. Furthermore, if you
create a new container and use User and Post via that new container,
the cache factory value for Post will be that of the old container and
not the new container.

This only is a problem if you expect to have Post be a different factory
in each container. In ember-cli, MODEL_FACTORY_INJECTIONS are enabled.
This means all factories are extended before being returned by the
container. So every time you have a new container, you have a new
factory (though it looks and behaves the same).

This change gets rid of that cache on meta. Instead it creates a
function that accepts the store and meta, and returns the relationship
object expected throughout Ember-Data. Two alternatives were considered:

  1. Always check against typeKey instead of instanceof, but this felt
like a dodge of the real problem and would leave the issue for others to
find, or
  2. Create a new first-class relationship object, but this would
be a far more invasive change, and would impact much of the codebase
that the current relationship object has already leaked through.

Additionally, the types were being cached on model class-level CPs. For
this patch, those have simply been marked cacheable(false). A later
patch should refactor the CPs out of the way, or at least re-write their
consumers to expect a relationship object without the factory itself
already fetched.