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/Region/Framework/Scenes/EventManager.cs

@@ -71,6 +71,7 @@ namespace OpenSim.Region.Framework.Scenes
         /// Triggered when a new client is added to the scene.
         /// </summary>
         /// <remarks>
+        /// This is triggered for both child and root agent client connections.
         /// Triggered before OnClientLogin.
         /// </remarks>
         public event OnNewClientDelegate OnNewClient;
@@ -191,7 +192,7 @@ namespace OpenSim.Region.Framework.Scenes
         public delegate void ClientClosed(UUID clientID, Scene scene);
 
         /// <summary>
-        /// Fired when a client is removed from a scene.
+        /// Fired when a client is removed from a scene whether it's a child or a root agent.
         /// </summary>
         /// <remarks>
         /// At the point of firing, the scene still contains the client's scene presence.