Restore performance of 304s after #10101

[bdraco]

The fix certainly handles the closings smartly, but I would be more worried about the additional overhead of the file openings. If cache is controlled well, many implementations are likely serving 304s much more than 200s. #10101 probably introduced a minor penalty there, but I don't see a better way to fix this issue. 🤷🏻‍♂️

Originally posted by @steverep in #8013 (comment)

[bdraco]

I think we could move the checks into the executor job as they are thread-safe already. Then return the state from executor job without the file open if its going to be a 304

[bdraco]

ie this block

        etag_value = f"{st.st_mtime_ns:x}-{st.st_size:x}"
        last_modified = st.st_mtime

        # https://www.rfc-editor.org/rfc/rfc9110#section-13.1.1-2
        ifmatch = request.if_match
        if ifmatch is not None and not self._etag_match(
            etag_value, ifmatch, weak=False
        ):
            return await self._precondition_failed(request)

        unmodsince = request.if_unmodified_since
        if (
            unmodsince is not None
            and ifmatch is None
            and st.st_mtime > unmodsince.timestamp()
        ):
            return await self._precondition_failed(request)

        # https://www.rfc-editor.org/rfc/rfc9110#section-13.1.2-2
        ifnonematch = request.if_none_match
        if ifnonematch is not None and self._etag_match(
            etag_value, ifnonematch, weak=True
        ):
            return await self._not_modified(request, etag_value, last_modified)

        modsince = request.if_modified_since
        if (
            modsince is not None
            and ifnonematch is None
            and st.st_mtime <= modsince.timestamp()
        ):
            return await self._not_modified(request, etag_value, last_modified)

would become something like

        etag_value = f"{st.st_mtime_ns:x}-{st.st_size:x}"
        last_modified = st.st_mtime

        # https://www.rfc-editor.org/rfc/rfc9110#section-13.1.1-2
        ifmatch = request.if_match
        if ifmatch is not None and not self._etag_match(
            etag_value, ifmatch, weak=False
        ):
            return FileResponseState.PRE_CONDITION_FAILED

        unmodsince = request.if_unmodified_since
        if (
            unmodsince is not None
            and ifmatch is None
            and st.st_mtime > unmodsince.timestamp()
        ):
            return FileResponseState.PRE_CONDITION_FAILED

        # https://www.rfc-editor.org/rfc/rfc9110#section-13.1.2-2
        ifnonematch = request.if_none_match
        if ifnonematch is not None and self._etag_match(
            etag_value, ifnonematch, weak=True
        ):
            return FileResponseState.NOT_MODIFIED

        modsince = request.if_modified_since
        if (
            modsince is not None
            and ifnonematch is None
            and st.st_mtime <= modsince.timestamp()
        ):
            return FileResponseState.NOT_MODIFIED