The CIMI spec does not permit providers setting properties on their
own. They are strictly for use by the client.
The 'entities' table now associates entries with CIMI::Model objects rather
than backend objects, and treats the DB as a proper look-aside cache.

This has the advantage that converting from backend objects is decoupled
from looking up the common attributes we store in the database (name,
description, properties)

The lookup of common attributes now happens when the model object is
constructed (at the point that the :id attribute is set, to be exact), and
can be saved with a call to its save method.