Add a QueryItem method to the inventory subsystem. Currently implemented for

MySQL only, stubs for the others. This allows updating the cache with a single
item from the database.

OpenSim/Data/MySQL/MySQLInventoryData.cs

@@ -539,6 +539,11 @@ namespace OpenSim.Data.MySQL
             }
         }
 
+        public InventoryItemBase queryInventoryItem(UUID itemID)
+        {
+            return getInventoryItem(itemID);
+        }
+
         /// <summary>
         /// Creates a new inventory folder
         /// </summary>