Exposed existing managed object fetching in MTLManagedObjectAdapter

The problem described in #281 is kind of blocking me right now, let's do something :) 

I still think the best option would be exposing a method of `MTLManagedObjectAdapter` for subclassing. This approach is used in this PR. I don't think this should be part of `<MTLManagedObjectSerializing>` because fetching is not entity's business, more like a task for some kind of cache or storage.

Any thoughts?