Add ability to delete key/value pairs during delta updates for Cueball and Curly

[thomas-kielbus]

It seems like implementing the deletion part during an update would be easy (simply skip keys in Cueball and do nothing for Curly). At this point, we probably need to use a flag in place of the Cueball value to indicate a key to delete. The domain builders could consider a key to be deleted when the value is null.