Added assets service method AssetsExist(), which returns whether the given list of assets exist.

This method is used to optimize sending assets with embedded assets: e.g., when a Hypergrid visitor takes an item into the inventory.

OpenSim/Data/IXAssetDataPlugin.cs

@@ -39,7 +39,7 @@ namespace OpenSim.Data
     {
         AssetBase GetAsset(UUID uuid);
         void StoreAsset(AssetBase asset);
-        bool ExistsAsset(UUID uuid);
+        bool[] AssetsExist(UUID[] uuids);
         List<AssetMetadata> FetchAssetMetadataSet(int start, int count);
         void Initialise(string connect);
         bool Delete(string id);