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/MSSQL/MSSQLInventoryData.cs

@@ -530,6 +530,11 @@ namespace OpenSim.Data.MSSQL
             }
         }
 
+        public InventoryItemBase queryInventoryItem(UUID itemID)
+        {
+            return null;
+        }
+
         /// <summary>
         /// Returns all activated gesture-items in the inventory of the specified avatar.
         /// </summary>