API: Support annotations on top-level nodes and mark the application.

Add a `nodeAnnotation` parameter to `createNode` to allow top-level
nodes to be tagged. Interpret the "application" annotation as
identifying the root of the application tree. Update the replication
data to include the tags.

This replaces the previous heuristic of treating the first node created
as the application root.

References #1272.
API: Add `kernel.deleteChild` to delete a node by name.

References #1272.
Create a global `clients.vwf` node and maintain one child per client.

The server creates the global node `http://vwf.example.com/clients.vwf`
on launch (independent of the application) then adds one child to it for
each client connected to the application instance. The child's name is
the client's moniker.

In the default implementation, neither the `clients.vwf` node nor its
children contain any additional data. It is expected that server
installations that have user data to share will make that available to
the application by creating the nodes with additional behaviors and
properties.

References #1272.