http: reset parser.incoming when server response is finished

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

Fixes: https://github.com/nodejs/node/issues/9668

PR-URL: https://github.com/nodejs/node/pull/28646


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