Simple GenServer callback to create a map w/ {key, value} pairs by looking at the filesystem

hydrate_state_from_filesystem.ex

  @impl GenServer
  def handle_call({:all}, _timeout, _state) do
    {:ok, files} = :file.list_dir(@database)
    state =
      Enum.map(files, fn (key) ->
        value =
          case File.read(file_name(@database, key)) do
            {:ok, contents} -> :erlang.binary_to_term(contents)
            _ -> nil
          end
        {key, value}
      end)
      |> Enum.into(%{})
    {:reply, state, state}
  end