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/Region/OptionalModules/World/NPC/NPCAvatar.cs

@@ -43,7 +43,6 @@ namespace OpenSim.Region.OptionalModules.World.NPC
         private readonly UUID m_uuid = UUID.Random();
         private readonly Scene m_scene;
 
-
         public NPCAvatar(string firstname, string lastname, Vector3 position, Scene scene)
         {
             m_firstname = firstname;
@@ -57,6 +56,8 @@ namespace OpenSim.Region.OptionalModules.World.NPC
             get { return m_scene; }
         }
 
+        public ISceneAgent SceneAgent { get { throw new NotImplementedException(); } }
+
         public void Say(string message)
         {
             SendOnChatFromClient(message, ChatTypeEnum.Say);
@@ -841,6 +842,8 @@ namespace OpenSim.Region.OptionalModules.World.NPC
 
         public void Start()
         {
+            // We never start the client, so always fail.
+            throw new NotImplementedException();
         }
         
         public void Stop()