Refactor: remove statewriter type and all callback for process start

This is a small prereq. for the runtime ( foreground mode ) and brings libcontainer to act more like a proper lib.  

From a commit: 

This temp. expands the Exec method's signature but adds a more robust
way to know when the container's process is actually released and begins
to run.  The network interfaces are not guaranteed to be up yet but this
provides a more accurate view with a single callback at this time.