async_hooks: make sure `.{en|dis}able() === this`

Make sure that `hook.enable()` and `hook.disable()` return `hook`
consistently, as the documentation indicates.

PR-URL: https://github.com/nodejs/node/pull/13418


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>