Ensure handlers run on a worker thread

This is to address the issue described in the comments for https://github.com/ptaoussanis/clojure-web-server-benchmarks/pull/17

It'll make handlers that hardly do anything slightly slower, but running handlers on the io threads can result in client starvation under load.
Merge pull request #4 from jcrossley3/patch-1

Ensure handlers run on a worker thread