Allow to assign Node.Name only once to avoid on-change subscriptions overhead

[SergeiPavlov]

Dumps show that Nodes' on-Name-Change subscriptions take > 11MB

But they are not used, because the Domain Model is immutable.
And the App does not use provided API to modify the built model.

In this PR I've added checking to ensure Node.Name is allowed to be assigned only once to non-null value.
It would be better to convert it into init setter, but will require massive refactoring.

Also:

• Node doesn't implement IChangeNotifier anymore
• Make Node.Name setter be internal

[botinko]

Sorry, couldnt follow for all of PR changes. Approve just in case.