refactor: simplify flush writer (#1066) #1601

Job	Run time
build-test-cohttp-bench (ubuntu-latest, 5.x, *.opam )	7m 32s
build-test-cohttp-async (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !c...	2m 58s
build-test-cohttp-async (ubuntu-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !c...	8m 13s
build-test-cohttp-eio (ubuntu-latest, 5.x, http.opam cohttp.opam cohttp-eio.opam )	4m 8s
build-test-default (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !cohttp...	2m 57s
build-test-cohttp-async (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !co...	2m 17s
build-test-cohttp-eio (macos-latest, 5.x, http.opam cohttp.opam cohttp-eio.opam )	4m 12s
build-test-default (ubuntu-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !cohttp...	7m 3s
build-test-cohttp-async (macos-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !co...	6m 39s
build-test-default (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !cohttp-...	2m 17s
build-test-default (macos-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !cohttp-...	5m 25s
	53m 41s

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