dns: implement {ttl: true} for dns.resolve4()

Add an option to retrieve the Time-To-Live of the A record.

PR-URL: https://github.com/nodejs/node/pull/9296
Refs: https://github.com/nodejs/node/issues/5893


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>