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.

Author: evantorrie

lib/_http_agent.js

@@ -212,7 +212,6 @@ Agent.prototype.createSocket = function createSocket(req, options, cb) {
 };
 
 function installListeners(agent, s, options) {
-
   function onFree() {
     debug('CLIENT socket onFree');
     agent.emit('free', s, options);
@@ -239,7 +238,7 @@ function installListeners(agent, s, options) {
     s.removeListener('agentRemove', onRemove);
   }
   s.on('agentRemove', onRemove);
-};
+}
 
 Agent.prototype.removeSocket = function removeSocket(s, options) {
   var name = this.getName(options);