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.

Backport-PR-URL: https://github.com/nodejs/node/pull/15500
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>

lib/_http_agent.js

@@ -206,9 +206,15 @@ Agent.prototype.createSocket = function(req, options, cb) {
     }
     self.sockets[name].push(s);
     debug('sockets', name, self.sockets[name].length);
+    installListeners(self, s, options);
+    cb(null, s);
+  }
+};
 
+function installListeners(agent, s, options) {
   function onFree() {
-      self.emit('free', s, options);
+    debug('CLIENT socket onFree');
+    agent.emit('free', s, options);
   }
   s.on('free', onFree);
 
@@ -217,7 +223,7 @@ Agent.prototype.createSocket = function(req, options, cb) {
     // This is the only place where sockets get removed from the Agent.
     // If you want to remove a socket from the pool, just close it.
     // All socket errors end in a close event anyway.
-      self.removeSocket(s, options);
+    agent.removeSocket(s, options);
   }
   s.on('close', onClose);
 
@@ -226,17 +232,15 @@ Agent.prototype.createSocket = function(req, options, cb) {
     // (defined by WebSockets) where we need to remove a socket from the
     // pool because it'll be locked up indefinitely
     debug('CLIENT socket onRemove');
-      self.removeSocket(s, options);
+    agent.removeSocket(s, options);
     s.removeListener('close', onClose);
     s.removeListener('free', onFree);
     s.removeListener('agentRemove', onRemove);
   }
   s.on('agentRemove', onRemove);
-    cb(null, s);
 }
-};
 
-Agent.prototype.removeSocket = function(s, options) {
+Agent.prototype.removeSocket = function removeSocket(s, options) {
   var name = this.getName(options);
   debug('removeSocket', name, 'writable:', s.writable);
   var sets = [this.sockets];