Start all queues in parallel on init

The midwife now starts queues using an async stream to parallelize
startup and minimize boot time for applications with many queues.

lib/oban/midwife.ex

@@ -15,7 +15,8 @@ defmodule Oban.Midwife do
     GenServer.start_link(__MODULE__, struct!(State, opts), name: name)
   end
 
-  @spec start_queue(Config.t(), Keyword.t()) :: DynamicSupervisor.on_start_child()
+  @spec start_queue(Config.t(), Keyword.t() | {String.t(), Keyword.t()}) ::
+          DynamicSupervisor.on_start_child()
   def start_queue(conf, opts) when is_list(opts) do
     queue =
       opts
@@ -33,6 +34,12 @@ defmodule Oban.Midwife do
     |> DynamicSupervisor.start_child({Queue.Supervisor, opts})
   end
 
+  def start_queue(conf, {queue, opts}) do
+    opts
+    |> Keyword.put(:queue, queue)
+    |> then(&start_queue(conf, &1))
+  end
+
   @spec stop_queue(Config.t(), atom() | String.t()) :: :ok | {:error, :not_found}
   def stop_queue(conf, queue) do
     case Registry.whereis(conf.name, {:queue, queue}) do
@@ -48,17 +55,13 @@ defmodule Oban.Midwife do
 
   @impl GenServer
   def init(state) do
-    start_all_queues(state.conf)
+    state.conf.queues
+    |> Task.async_stream(fn opts -> {:ok, _} = start_queue(state.conf, opts) end)
+    |> Stream.run()
 
     {:ok, state, {:continue, :start}}
   end
 
-  defp start_all_queues(conf) do
-    for {queue, opts} <- conf.queues do
-      {:ok, _pid} = start_queue(conf, Keyword.put(opts, :queue, queue))
-    end
-  end
-
   @impl GenServer
   def handle_continue(:start, %State{conf: conf} = state) do
     Notifier.listen(conf.name, :signal)