feat(cohttp-async): add [close_finished] in [Connection] #1590

Job	Run time
build-test-cohttp-async (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !c...	3m 4s
build-test-default (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !cohttp...	3m 6s
build-test-cohttp-async (ubuntu-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !c...	9m 5s
build-test-default (ubuntu-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !cohttp...	8m 15s
build-test-cohttp-async (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !co...	1m 54s
build-test-cohttp-async (macos-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !co...	6m 44s
build-test-default (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !cohttp-...	2m 12s
build-test-cohttp-bench (ubuntu-latest, 5.x, *.opam )	7m 6s
build-test-default (macos-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !cohttp-...	5m 19s
build-test-cohttp-eio (ubuntu-latest, 5.x, http.opam cohttp.opam cohttp-eio.opam )	4m 58s
build-test-cohttp-eio (macos-latest, 5.x, http.opam cohttp.opam cohttp-eio.opam )	4m 4s
	55m 47s

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