Add some asset cache plumbing. Change the generic cache from UUID to string

keys to allow caching the new crop of URI identified objects.

OpenSim/Region/Framework/Interfaces/IImprovedAssetCache.cs

@@ -25,9 +25,15 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+using OpenSim.Framework;
+
 namespace OpenSim.Region.Framework.Interfaces
 {
     public interface IImprovedAssetCache
     {
+        void Cache(AssetBase asset);
+        AssetBase Get(string id);
+        void Expire(string id);
+        void Clear();
     }
 }