New release deployment downtime

[devgianlu]

We need a way to assure that new version deployment doesn't interfere with ongoing games. Shutting down the server isn't a great solution when someone will use this. A nice solution would be not allowing new games to be created, but only ongoing ones to end (in a certain amount of time, if someone doesn't end a game in X minutes, kill him anyway). Displaying a banner would be nice. Once the server has no more games, shut it down, deploy and done.

• Set preparing for shutdown
• Block some requests
• Set timer to force shutdown
• Listen for when we're ready to shutdown