async_wrap: add parent uid to init hook

When the parent uid is required it is not necessary to store the uid in
the parent handle object.

PR-URL: https://github.com/nodejs/node/pull/4600


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>