Combines multiple stores. Reads return the result from the first store, writes go to all stores.

@example Add `Moneta::Stack` to proxy stack

  Moneta.build do
    use(:Stack) do
      add { adapter :Redis }
      add { adapter :File, :dir => 'data' }
      add { adapter :File, :dir => 'replicate' }
    end
  end

@api public