Send only ids for subsets

[Arthur Valadares]

Since subsets never have more information than the original set object, a subset can essentially be represented as a list of ids, as long as the client also subscribes to the parent set as well.

A suggestion was for the server to send a dictionary with primary keys as ids and set an empty dictionary as a value when the server wishes the client to use its local copy of that object.

[Rohan Achar]

This is done in part in checkin d39539e.
Only modifications to dimensions, and the corresponding ids are sent during update.