http: fix timeout reset after keep-alive timeout

Fix the logic of resetting the socket timeout of keep-alive HTTP
connections and add two tests:

* `test-http-server-keep-alive-timeout-slow-server` is a regression test
  for GH-13391.  It ensures that the server-side keep-alive timeout will
  not fire during processing of a request.

* `test-http-server-keep-alive-timeout-slow-client-headers` ensures that
  the regular socket timeout is restored as soon as a client starts
  sending a new request, not as soon as the whole message is received,
  so that the keep-alive timeout will not fire while, e.g., the client
  is sending large cookies.

Refs: https://github.com/nodejs/node/pull/2534
Fixes: https://github.com/nodejs/node/issues/13391
PR-URL: https://github.com/nodejs/node/pull/13549


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>