add some more http benchmarks

[graingert]

Currently there's tornado_http but there's loads of high quality benchmarks for asyncio clients and servers

https://magic.io/blog/uvloop-blazing-fast-python-networking/

I also want to see if it's possible to make SelectorEventLoop perform faster than uvloop

[mdboom]

I think this is a great idea, especially for some combination of micro- and macrobenchmarks.

Most of what we have already in the suite is CPU-bound, and I wonder if some of this http benchmarks might be I/O bound. First, it would be good just to know that. But secondly, we may want to take care about how they roll up into aggregate numbers (probably through use of the aggregation-by-tagging that recently landed in pyperf).

[graingert]

My understanding is that the uvloop benchmarks were already very carefully designed to eliminate IO bound code