Add ability to set maximum size of a resource for mod_pagespeed to optimize #501

From a user report:

if there is a request for a huge file then mod pagespeed goes for a spin and 
the RAM usage bloats up. Is there a way to tune this, or ask modpagespeed to 
not process if the response size is bigger than a certain limit. Currently we 
are handling by having a cron that kill any httpd process that exceeds a 
certain RAM usage size.