Add naive implementation of controlled incoming HG attachments to manage load.

Instead of processing all incoming attachment scene object concurrently, process them consecutively to eliminate potential overload from this source.
This is a naive implementation because it does not currently account for slow foreign asset services.
Although it may take longer, this approach may also improve attachment visibility for HG avatars
since the scene object is now always added to the scene after receiving assets from the foreign service and not before.

OpenSim/Region/Framework/Interfaces/IEntityTransferModule.cs

@@ -98,6 +98,8 @@ namespace OpenSim.Region.Framework.Interfaces
         void Cross(SceneObjectGroup sog, Vector3 position, bool silent);
 
         ScenePresence CrossAgentToNewRegionAsync(ScenePresence agent, Vector3 pos, GridRegion neighbourRegion, bool isFlying, string version);
+
+        bool HandleIncomingSceneObject(SceneObjectGroup so, Vector3 newPosition);
     }
 
     public interface IUserAgentVerificationModule