Keep Plug connection open

plug.ex

defmodule AlivePlug do
  import Plug.Conn

  def init(opts) do
    opts
  end

  def call(conn, _opts) do
    conn = send_chunked(conn, 200)
    # send initial chunk
    chunk(conn, "Init")

    pid = start_thread
    # create 10 async processes which does somthing for 5 seconds and then sends result via chunk
    1..10
    |> Enum.each(fn num -> 
      send pid, {conn, num}
    end)

    # send End as a symbol of last chunk
    chunk(conn, "End")
    conn
  end

  defp start_thread, do: spawn_link(fn -> thread_listener end)
  defp thread_listener do
    receive do
      {conn, num} ->
        :timer.sleep(5000)
        # problem is here :(
        # chunk returns {:error, :closed}
        {:error, :closed} = chunk(conn, "hey")
        thread_listener
      _ -> 
        thread_listener
    end
  end
end