Start Peer and Stager after queue supervisor

The queue supervisor blocks shutdown to give jobs time to shut down
gracefully. During that time, the Peer could obtain or retain leadership
despite all of the plugins having stopped. Now the Peer and Stager
(which is only active on the leader) stop before the queue supervisor.

lib/oban.ex

@@ -400,10 +400,10 @@ defmodule Oban do
  def init(%Config{plugins: plugins} = conf) do
  children = [
  {Notifier, conf: conf, name: Registry.via(conf.name, Notifier)},
- {Peer, conf: conf, name: Registry.via(conf.name, Peer)},
- {Stager, conf: conf, name: Registry.via(conf.name, Stager)},
  {DynamicSupervisor, name: Registry.via(conf.name, Foreman), strategy: :one_for_one},
- {Midwife, conf: conf, name: Registry.via(conf.name, Midwife)}
+ {Peer, conf: conf, name: Registry.via(conf.name, Peer)},
+ {Midwife, conf: conf, name: Registry.via(conf.name, Midwife)},
+ {Stager, conf: conf, name: Registry.via(conf.name, Stager)}
  ]
 
  children = children ++ Enum.map(plugins, &plugin_child_spec(&1, conf))