Add LimitedStream benchmark

[carltongibson]

Hi @deepakdinesh1123 @smithdc1.

Have a look at the ticket here: https://code.djangoproject.com/ticket/33865

There's a proposal to optimise LimitedStream (which wraps the WSGI request body input stream to ensure we can't read into other requests...).

It might be nice to quickly add a benchmark for it, and then be able to say, "Look here's main, and here's the PR". Are we there yet?

What do you think?