Smart merge of annotated AgId parts on top of Cayenne model

[andrus]

Per #580 , we fixed merging of annotated attributes and relationships on top of the model created by CayenneAgEntityCompiler. Overrides of IDs is still done the "old" way - if @AgId is present, we replace Cayenne-specific ID parts with generic ID parts. This may cause subtle bugs in the code.

Should implement a smart merge instead. See CayenneAgEntityBuilder.buildAnnotatedProperties

While we are at it, we may re-implement attributes and relationships merge via AgEntityOverlay, and proper unit tests for the whole merge mechanism