Fix leakage of internal class hierarchy

Fix leakage of internal class hierarchy

Allocate the InternalClass instances in a memory pool and delete them
all in one shot at the end. The memory pool won't call the destructor,
so we have to introduce our own recursive destroy() method, that will
free the memory allocated.

Change-Id: I507a893475181f765acf545b5cd2bfccc8c29747
Reviewed-by: Lars Knoll <lars.knoll@digia.com>