http: fix backport of Slowloris headers

The backport of 618eebdd17 was
not complete, and the starting time to parse the headers was not reset.

Fixes: #24760

Author: mcollina

lib/_http_server.js

@@ -481,6 +481,9 @@ function connectionListener(socket) {
   function parserOnIncoming(req, shouldKeepAlive) {
     incoming.push(req);
 
+    // Set to zero to communicate that we have finished parsing.
+    socket.parser.parsingHeadersStart = 0;
+
     // If the writable end isn't consuming, then stop reading
     // so that we don't become overwhelmed by a flood of
     // pipelined requests that may never be resolved.