cluster: return worker reference from disconnect()

Changes disconnect() to return a refererence to the worker.
This will enable method chaining such as

    worker.disconnect().once('disconnect', doThis);

PR-URL: https://github.com/nodejs/node/pull/10019


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>