On startup, start scenes after we're set up all local scenes, rather than starting scenes before others have been created.

This aims to avoid a race condition where scenes could look to inform neighbours that they were up before those neighbours had been created.
http://opensimulator.org/mantis/view.php?id=6618

OpenSim/Region/Framework/Scenes/SceneBase.cs

@@ -561,6 +561,10 @@ namespace OpenSim.Region.Framework.Scenes
             get { return false; }
         }
 
+        public virtual void Start()
+        {
+        }
+
         public void Restart()
         {
             // This has to be here to fire the event