orm: Implement object caching

Ensure that for a model class name and primary key, there is only one object
floating around in PHP runtime. In doing so, ensure that models loaded from
the database are not duplicated in memory. Instead, check the cache when
creating objects to see if the object has already been loaded. If so, use
the loaded model rather than a new instance. This ensures that updates to a
model object are reflected consistently.

Also, short circuit lookups so that the cached version is retrieved without
consulting the database.