http: Change free sockets behavior to LIFO from FIFO.

Sockets are added to the free list with .push() but they were
being removed with .shift().  This meant the sockets where being
removed in FIFO order, but this changes it to LIFO.  Since older
sockets may be closed based due to inactivity on the server it is
more likely that a socket that is recently used will be able to
successfully process the next request.

Rather than destroying the last used socket destroy
the oldest socket in the free list in push() on the
last recently used socket.

lib/_http_agent.js

@@ -112,11 +112,16 @@ function Agent(options) {
         if (this.sockets[name])
           count += this.sockets[name].length;
 
-        if (count > this.maxSockets || freeLen >= this.maxFreeSockets) {
-          socket.destroy();
-        } else if (this.keepSocketAlive(socket)) {
-          freeSockets = freeSockets || [];
-          this.freeSockets[name] = freeSockets;
+        if (this.keepSocketAlive(socket) &&
+            this.maxFreeSockets > 0 &&
+            count <= this.maxSockets) {
+          if (freeLen >= this.maxFreeSockets) {
+            const oldest = this.freeSockets[name].shift();
+            oldest.destroy();
+          } else {
+            freeSockets = freeSockets || [];
+            this.freeSockets[name] = freeSockets;
+          }
           socket[async_id_symbol] = -1;
           socket._httpMessage = null;
           this.removeSocket(socket, options);
@@ -207,7 +212,7 @@ Agent.prototype.addRequest = function addRequest(req, options, port/* legacy */,
 
   if (freeLen) {
     // We have a free socket, so use that.
-    const socket = this.freeSockets[name].shift();
+    const socket = this.freeSockets[name].pop();
     // Guard against an uninitialized or user supplied Socket.
     const handle = socket._handle;
     if (handle && typeof handle.asyncReset === 'function') {