Disconnect (DOM)DataTreeIdentifier from serialization

Both Binding DataTreeIdentifier and DOMDataTreeIdentifier are used to address datastore content.

We would be hard-pressed to come up with better names, so for NMDA evolution we really need to start thinking about how to repurpose them.

Since both are HierarchicalIdentifiers, they are are also serializable and they do not have serialization proxies.

We need to switch them to use serialization proxies, so they can be evolved. The flip side of that is that we will also have more efficient serialization.