Subzero: Manage each Cfg as a std::unique_ptr<Cfg>.

The Cfg::create() method now returns a unique_ptr.  Once the parser fully builds the Cfg, it is released onto the work queue, and then acquired and ultimately deleted by the translator thread.

BUG= none
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/892063002