Eng 6530 txn not span buffers

Can we make this a tristate? So the first time you move it to its own buffer, the next time you move it to a 45 megabyte buffer, and then after that you throw an exception and roll back. We might as well support up to the largest transaction size supportable and we need to go bigger than 2 megabytes for updates.

You will need to test and make sure a big transaction can propagate downstream. I'll bet some things might mess up and need tweaking.

All this functionality also needs to be specific to DR. TupleStreamBase is shared with export. You could delegate to a subclass or have a method that returns the subclasses policy.