Using worker threads instead of httpd threads means that we, not libevent,
decide which requests get handled by which threads. This means that we can
make sure blocking requests (e.g. volume changes) don't get in the way of
realtime(ish) stuff like mp3 streaming.
Includes refactor of httpd_stream_file() since it was a bit of a monster.
Despite that it seemingly supports multithreading it picks threads that are
busy, and not sure how that can be prevented. Also libevhtp is currently
unmaintained.
