Use multiple hackney pools

* federation (ap, salmon)
* media (rich media, media proxy)
* upload (uploader proxy)

Each "part" will stop fighting others ones -- a huge federation outbound
could before make the media proxy fail to checkout a connection in time.

splitted media and uploaded media for the good reason than an upload
pool will have all connections to the same host (the uploader upstream).
it also has a longer default retention period for connections.

lib/pleroma/uploaders/mdii.ex

@@ -24,7 +24,8 @@ defmodule Pleroma.Uploaders.MDII do
     extension = String.split(upload.name, ".") |> List.last()
     query = "#{cgi}?#{extension}"
 
-    with {:ok, %{status: 200, body: body}} <- @httpoison.post(query, file_data) do
+    with {:ok, %{status: 200, body: body}} <-
+           @httpoison.post(query, file_data, adapter: [pool: :default]) do
       remote_file_name = String.split(body) |> List.first()
       public_url = "#{files}/#{remote_file_name}.#{extension}"
       {:ok, {:url, public_url}}