Publisher: check out a connection for inserting publish_one jobs

Related to #1474, federation of one post on my istance creates in
best-case 360 jobs, so if they for some reason take a while to insert,
it will exhaust the connection pool. This fixes it by checking out one
dedicated connection for inserting them.

lib/pleroma/web/activity_pub/publisher.ex

@@ -9,6 +9,7 @@ defmodule Pleroma.Web.ActivityPub.Publisher do
   alias Pleroma.HTTP
   alias Pleroma.Instances
   alias Pleroma.Object
+  alias Pleroma.Repo
   alias Pleroma.User
   alias Pleroma.Web.ActivityPub.Relay
   alias Pleroma.Web.ActivityPub.Transmogrifier
@@ -188,12 +189,15 @@ def publish(%User{} = actor, %{data: %{"bcc" => bcc}} = activity)
 
     recipients = recipients(actor, activity)
 
+    inboxes =
       recipients
       |> Enum.filter(&User.ap_enabled?/1)
       |> Enum.map(fn %{source_data: data} -> data["inbox"] end)
       |> Enum.filter(fn inbox -> should_federate?(inbox, public) end)
       |> Instances.filter_reachable()
-    |> Enum.each(fn {inbox, unreachable_since} ->
+
+    Repo.checkout(fn ->
+      Enum.each(inboxes, fn {inbox, unreachable_since} ->
         %User{ap_id: ap_id} =
           Enum.find(recipients, fn %{source_data: data} -> data["inbox"] == inbox end)
 
@@ -214,6 +218,7 @@ def publish(%User{} = actor, %{data: %{"bcc" => bcc}} = activity)
           unreachable_since: unreachable_since
         })
       end)
+    end)
   end
 
   @doc """