Make shutdown in OnApplicationQuit optional

[danielchipman]

Is your feature request related to a problem? Please describe. I use the Application.wantsToQuit callback to allow me to gracefully terminate certain things before the app concludes, such as updating Lobby info, etc. I'd also like to use it to gracefully let clients know that the server has purposefully terminated, but OnApplicationQuit() triggers and shuts down the network before any of my code can run.

Describe the solution you'd like An option on NetworkManager to disable automatic shutdown in response to OnApplicationQuit so that I may handle it in a custom manner.