Lock the world so we don't cause nastiness and crashes

src/world/World.cs

@@ -39,9 +39,11 @@ namespace OpenSim.world
 	}
 	
 	public void DoStuff() {
+		lock(this) {
 			physics.DoStuff(this);
 			this.Update();
 		}
+	}
 
 	public void Update() {
             foreach (libsecondlife.LLUUID UUID in Entities.Keys)