Add nginx+uWSGI variations to tests: uwsgi, wsgi, bottle, flask

This was the result for the json 256-concurrency benchmark:

uwsgi: 30% faster
wsgi: 10% faster
bottle: ~20% faster
flask: ~7% *SLOWER*

I also tweaked the existing uwsgi test for a 30% gain by using --http-processes. BTW, the uwsgi test uses --gevent even though the code does not use gevent. This is causing a perf hit, but perhaps the point of the test is to test gevent support, so I didn't remove it.

It is interesting that --thunder-lock actually decreased performance in some cases, thus I didn't use it.

And it's super interesting that flask got slower with nginx+uWSGI compared to gunicorn. I'm not sure what's causing that, but perhaps we'll see more interesting results (for even more realistic tests like fortune) when the next test run is done.

CC: @methane, #475