After seeing sdague do his happy dance over trunk working "the best he has ever seen". I'm not sure I should be doing this commit, but oh well.

So anyway, it moves the Asset downloading (packet sending) to a module (AssetDownloadModule). 
So now at last, AssetCache should be just dealing with fetching assets from the asset server and caching them.

OpenSim/Region/Examples/SimpleApp/MyNpcCharacter.cs

@@ -131,6 +131,7 @@ namespace SimpleApp
         public event RezScript OnRezScript;
         public event UpdateTaskInventory OnUpdateTaskInventory;
         public event RemoveTaskInventory OnRemoveTaskItem;
+        public event RequestAsset OnRequestAsset;
 
         public event UUIDNameRequest OnNameFromUUIDRequest;