NamedList remove improvements

From API perspective to remove from NamedList we need to nullify a particular element in the list.

On the Storage level it turns into removing all keys sitting under this particular element.

Configuration engine should be responsible for cleaning up all necessary keys from Storage. Notifications should be aware of removing from NamedLists as well (e.g. one notification about removing the element from NL instead of bunch of notifications about each NL's element's field).