Clean stale DeviceConnections (#1747)

I noticed in the DB that we have several old/stale device connections. 

Upon looking into this more, I realised that when we added the
`DeviceConnection` schema and relationships, we didn't add a way to
clean the stale connections, which might linger due to unforeseen
circumstances, like the app crashing.

lib/nerves_hub/devices/connections.ex

@@ -128,4 +128,20 @@ defmodule NervesHub.Devices.Connections do
     |> distinct(:device_id)
     |> order_by([:device_id, desc: :last_seen_at])
   end
+
+  def clean_stale_connections() do
+    interval = Application.get_env(:nerves_hub, :device_last_seen_update_interval_minutes)
+    a_minute_ago = DateTime.shift(DateTime.utc_now(), minute: -(interval + 1))
+
+    DeviceConnection
+    |> where(status: :connected)
+    |> where([d], d.last_seen_at < ^a_minute_ago)
+    |> Repo.update_all(
+      set: [
+        status: :disconnected,
+        disconnected_at: DateTime.utc_now(),
+        disconnected_reason: "Stale connection"
+      ]
+    )
+  end
 end

lib/nerves_hub/workers/clean_device_connection_states.ex

@@ -3,9 +3,13 @@ defmodule NervesHub.Workers.CleanDeviceConnectionStates do
     max_attempts: 5,
     queue: :device
 
+  alias NervesHub.Devices
+  alias NervesHub.Devices.Connections
+
   @impl true
   def perform(_) do
-    NervesHub.Devices.clean_connection_states()
+    Devices.clean_connection_states()
+    Connections.clean_stale_connections()
 
     :ok
   end