MDL-47740 question engine: let qs update variant later if appropriate

Variant has two purposes. First to determine which version of the
question the student will see. Hence it is used to set up the state
of the quetsion when the question is started. Then the internal state of
the question is saved in the first step.

Once that has been done, the variant number is purely informative, and
just used to break down the statistics.

In some cases (the one I have in mind is qtype_opaque) then there is a
complex randomisation process, which may lead to several inital variant
numbers acutally giving the same version of the question. In this case
it is nice if the question can update the stored variant number, to make
the statistics more meaningful.