serf: force threaded fetches

This should prevent stalling the ngx event loop when using the serf fetcher, and really speeds up  response times for me when there are a lot of uncached resources to fetch. 

Possibly fixes another cause of hanging pages by eliminating a deadlock situations which can only be resolved by timeouts.

Possibly related: https://github.com/pagespeed/ngx_pagespeed/issues/310