refactor: rename Scene.IncomingCloseAgent() to CloseAgent() in order to make it clear that all non-clientstack callers should be using this rather than RemoveClient() in order to step through the ScenePresence state machine properly.

Adds IScene.CloseAgent() to replace RemoveClient()
refactor: Rename Scene.AddNewClient() to AddNewAgent() to make it obvious in the code that this is symmetric with CloseAgent()