Eng 6607 - Add support for truncate table to DR2

It's a reasonable thing to assert on, so the question is what does it mean to have an undo quantum?

Right now executor context doesn't have a NULL pointer when an undo quantum is released, in fact there is a dangling pointer that could point to a deallocated undo quantum. I can fix that by having the code that releases undo quantum also null out the pointer and then you can assert whether the EE thinks a transaction is still running.