http: Change agent free socket behavior

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 +112,15 @@ function Agent(options) {
         if (this.sockets[name])
           count += this.sockets[name].length;
 
-        if (count > this.maxSockets || freeLen >= this.maxFreeSockets) {
+        if (count > this.maxSockets) {
           socket.destroy();
         } else if (this.keepSocketAlive(socket)) {
-          freeSockets = freeSockets || [];
-          this.freeSockets[name] = freeSockets;
+          if (freeLen >= this.maxFreeSockets) {
+            this.freeSockets[name].shift().destroy();
+          } else {
+            freeSockets = freeSockets || [];
+            this.freeSockets[name] = freeSockets;
+          }
           socket[async_id_symbol] = -1;
           socket._httpMessage = null;
           this.removeSocket(socket, options);