net: require 'dns' only once

Avoid unnecessary calls to require('dns') by caching the result of the
first one.

PR-URL: https://github.com/nodejs/node/pull/12342


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>