Major request handler performance improvement. Benchmarks suggest number of
requests/s improved by more than 50% for common GET requests!
We now avoid queuing, buffering and wrapping incoming requests in promises
when we're below limits and instead can directly process common requests.
(#291, #292, #293, #294 and #296 by @clue)

Fix: Fix concurrent invoking next middleware request handlers
(#293 by @clue)

Small code improvements
(#286 by @seregazhuk)

Improve test suite to be less fragile when using ext-event and
fix test suite forward compatibility with upcoming EventLoop releases
(#288 and #290 by @clue)