[v6.x] http: fix backport of Slowris 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.
Fixes: https://github.com/nodejs/node/issues/24760
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines