http: refactor responseKeepAlive()

This tries to simplify the code and make it easier to understand. Took
me a while to get my head around the previous implementation.

Also minor perf improvement.

PR-URL: https://github.com/nodejs/node/pull/28700


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>