Provide a way to force the queue write strategy, again

[nox]

Once upon a time, there was a way to force Hyper to use the queue write strategy.

This functionality was then removed from Hyper 0.14.0.

Unfortunately, the heuristic to decide whether to use flatten or queue strategy has its limits. In my case, I need to buffer the output of an egress HttpBody implementation until a few MBs, and then I pass that big buffer to Hyper to write it over a h1 stream. When I pass it, Hyper is using the flatten strategy, so it ends up copying the entire big buffer again.

I'll make a PR to restore the builder setter to force queue strategy.