util: delay creation of debug context

We need the debug context to be able to inspect promises. However, this is
very expensive and should not be done on default startup.

PR-URL: https://github.com/nodejs/io.js/pull/2248


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Christopher Monsanto <chris@monsan.to>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>