http: enable keepalive / connection pooling (closes #336)

lib/pleroma/http/http.ex

@@ -19,9 +19,10 @@ defp process_sni_options(options, url) do
     end
   end
 
-  def process_request_options(options) do
+  defp process_request_options(options) do
     config = Application.get_env(:pleroma, :http, [])
     proxy = Keyword.get(config, :proxy_url, nil)
+    options = options ++ [hackney: [pool: :default]]
 
     case proxy do
       nil -> options