lib: return this from net.Socket.end()

PR-URL: https://github.com/nodejs/node/pull/13481


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Brian White <mscdex@mscdex.net>