cluster: remove bind() and self

This commit removes the use of self and bind() from the cluster
module in favor of arrow functions.

PR-URL: https://github.com/nodejs/node/pull/7710


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>