Simplify friends caching by only doing this for root agents - no functions require caching for child agents.

This allows us to avoid unnecessary multiple calls to the friends service.
All friends functions originate from the root agent and only go to other root agents in existing code.
This also allows us to eliminate complex ref counting.

OpenSim/Framework/IClientAPI.cs

@@ -710,7 +710,7 @@ namespace OpenSim.Framework
         /// 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; }
+        ISceneAgent SceneAgent { get; set; }
 
         UUID SessionId { get; }