trunk-tracking: take advantage of new functionality in system/

SystemRewriteDriverFactory was expanded by r3366 to do a lot of what
NgxRewriteDriverFactory used to have to do.  Take advantage of this and
simplify our code.

This depends on changes to `pthread_shared_mem` (the ones Otto made in #179 when forking it) that are under review for mod_pagespeed and aren't pushed to svn yet, so it won't pass tests until that goes in.