async_wrap: add uid to all asyncWrap hooks

By doing this users can use a Map object for storing information instead
of modifying the 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>