Add a 'Shutting down' state to the gameserver.

Tell the masterserver about it, so it can quickly remove games
from the list when they are finished, rather than waiting for the
5 minute TTL to expire.

OpenRA.Mods.RA/ServerTraits/LobbyCommands.cs

@@ -39,7 +39,7 @@ namespace OpenRA.Mods.RA.Server
 
 		public static bool ValidateCommand(S server, Connection conn, Session.Client client, string cmd)
 		{
-			if (server.GameStarted)
+			if (server.State == ServerState.GameStarted)
 			{
 				server.SendChatTo(conn, "Cannot change state when game started. ({0})".F(cmd));
 				return false;