child_process: refactor self=this in socket_list

The socket list module (used by child_process) currently uses the
`var self = this;` pattern for context in several places, this PR
replaces this with arrow functions or passing a parameter in where
appropriate.

Note that the `var self = this` in the _request is intentioanlly
left in place since it is not trivial to refactor it and the current
pattern isn't bad given the use case.

PR-URL: https://github.com/nodejs/node/pull/5860


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>