feat(expiry): add ttl sweep process

lib/excache/expiry.ex

@@ -0,0 +1,23 @@
+defmodule Excache.Expiry do
+  @moduledoc false
+
+  use GenServer
+
+  def start_link(_arg), do: GenServer.start_link(__MODULE__, nil, name: __MODULE__)
+
+  @impl true
+  def init(_arg) do
+    interval = Application.get_env(:excache, :sweep_interval, 1000)
+    schedule(interval)
+    {:ok, interval}
+  end
+
+  @impl true
+  def handle_info(:sweep, interval) do
+    Excache.Store.sweep() |> Enum.each(&Excache.Notify.expired/1)
+    schedule(interval)
+    {:noreply, interval}
+  end
+
+  defp schedule(interval), do: Process.send_after(self(), :sweep, interval)
+end