Redesign of AgEntity ID model - allow relationships as part of id

[andrus]

The recent work on exposing Ag metadata (#459) highlighted a number of shortcomings of the current entity metadata model in regards to ID handling, such as leaking of DB column names in IDs (#463).

And here is another one - logically from the Agrest perspective an ID can be comprised from a combination of attributes and relationships, but currently we can represent it only as a set of attributes. This spills over to things like ID handling in EntityUpdate, etc.

This task is an attempt to straighten the ID model of AgEntity:

• Introduce a superinterface of AgAttribute and AgRelationship - AgProperty
• Model ID as a Set<AgProperty>
• ID attributes and relationships should be present in the corresponding property collections (currently attributes returned from AgEntity.getIds() are NOT present in AgEntity.getAttributes())

The hope is that this would allow to fix the inconsistencies per #463, ID representation in EntityUpdate, etc. and generally present a consistent universal ID model not tied to the database structure.

Upgrade Notes

• Now @AgId can be combined with @AgAttribute or @AgRelationship over the same method. Standalone @AgId is resolved as @AgAttribute (this is backwards compatible).

[andrus]

This is too big of a departure from the current ID model. Will not be pursuing this, and will look for alternative solutions to ID issues.