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 nodejsGH-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-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: nodejs#2534
Fixes: nodejs#13391