Nested ROP contexts and relationships

[Andrus Adamchik]

I am in the middle of debugging a problem with nested ROP contexts  
losing arc changes when committing to a parent context. Since I was  
not involved in the ROP nested context work, I figured I'd post my  
thoughts here before I start changing the code.

I noticed that per CAY-1119, there is a special subclass of  
ChildDiffLoader called CayenneContextChildDiffLoader that calls  
'propertyChanged' on the parent context after syncing a simple  
property change. It seems like we need to do the same for  
relationships as well, to record arc changes in the parent diff list.

Andrey, do you have any comments on that? I wonder if it was omitted  
intentionally. I will open a Jira (I think we don't have one for  
this), and add some tests with various relationships, but before I dig  
any deeper figured I'll need a sanity check.

Thanks,
Andrus