http: reset parser.incoming when server response is finished

This resolves a memory leak for keep-alive connections with a naïve
approach.

Fixes: #9668

PR-URL: #28646
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/_http_server.js

@@ -621,6 +621,8 @@ function resOnFinish(req, res, socket, state, server) {
   assert(state.incoming.length === 0 || state.incoming[0] === req);
 
   state.incoming.shift();
+  // Reset the .incoming property so that the request object can be gc'ed.
+  if (socket.parser) socket.parser.incoming = null;
 
   // If the user never called req.read(), and didn't pipe() or
   // .resume() or .on('data'), then we call req._dump() so that the