http: eliminate capture of ClientRequest in Agent

Keepalive sockets that are returned to the agent's freesocket pool were
previously capturing a reference to the ClientRequest that initiated the
request.

This commit eliminates that by moving the installation of the socket
listeners to a different function.

PR-URL: https://github.com/nodejs/node/pull/10134


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>