On a new client circuit, send the initial reply ack to let the client know it's live before sending other data.

This means that avatar/appearance data of other avatars and scene objects for a client will be sent after the ack rather than possibly before.
This may stop some avatars appearing grey on login.
This introduces a new OpenSim.Framework.ISceneAgent to accompany the existing OpenSim.Framework.ISceneObject and ISceneEntity
This allows IClientAPI to handle this as it can't reference OpenSim.Region.Framework.Interfaces

OpenSim/Framework/IClientAPI.cs

@@ -702,6 +702,12 @@ namespace OpenSim.Framework
 
         UUID AgentId { get; }
 
+        /// <summary>
+        /// The scene agent for this client.  This will only be set if the client has an agent in a scene (i.e. if it
+        /// is connected).
+        /// </summary>
+        ISceneAgent SceneAgent { get; }
+
         UUID SessionId { get; }
 
         UUID SecureSessionId { get; }