http: fix backport of Slowloris headers

The backport of https://github.com/nodejs/node/commit/618eebdd17 was
not complete, and the starting time to parse the headers was not reset.

PR-URL: https://github.com/nodejs/node/pull/24796
Fixes: https://github.com/nodejs/node/issues/24760


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>