Dynamically creating it through Req_MakeTop() would further complicate
rollbacks.

The memory overhead is basically identical to embedding struct reqtop
into struct req, except that, for ESI, we have the (struct req).top
point to the top request's struct reqtop.

With this commit, tests/r02849.vtc and tests/r03003.vtc are failing as
excpected. While this may impose issues with git bisect, I still think
that this extra commit helps clarity.