Issue 609: Enable and disable networked objects

Repository: miragenet/mirage

Issue body:
As a game dev,  I want to enable and disable networked objects so that I can control when user can see them.

# Explanation
Currently Mirage takes over enable/disable.  All scene objects get disabled when the game starts and are enabled when the user connects.   
The user therefore cannot control when objects are enabled/disabled as it interferes with the scene object mechanism.
This also makes for a very complicated way to control objects.

discussions about NetworkScenePostProcssing: #608
- this feature is left over from UNET, hard to know the reason for the initial feature

# Acceptance criteria
When a spawned object is disabled in the server,  Mirage disables it in the client
When a spawned object is enabled in the server, Mirage enables it in the client
When the user starts the game,  the scene objects should not be visible until he connects to the server
