Fix performance issue with DS.Model.relationshipsByName

This PR addresses a performance issue with ``DS.Model.relationshipsByName``.

I noticed after profiling my application that on large results that a lot of time is spent in the ``relationshipsByName`` computed property which is called by ``typeForRelationship``.

Despite what the comments say about those properties becoming 'frozen', when ``typeForRelationship`` calls ``get(this, 'relationshipsByName')``  the name/relationship-descriptor map is rebuilt every time, adding considerable overhead to response processing.

By removing ``.cacheable(false)`` from the computed property, the map is now only every built once per model class **and halves the load time for ~300 models** (from ~2000ms down to ~1000ms) on a high end laptop, so the benefit on mobile devices or larger result sets would probably be greater.

I have read that the ``.caheable()`` property modifier has been removed in recent Ember commits, and probably ``volatile()`` should be used where required instead.  There may be other places in Ember Data that use deprecated CP modifiers, but I haven't had time to do an audit.

All tests pass, and given the contract in the documentation that such properties are supposed to be frozen, it seems like a valid change and a dramatic performance improvement.