HTTP: radically simplify pool checkin/checkout

Use a custom tesla middleware instead of adapter helper function +
custom redirect middleware.

This will also fix "Client died before releasing the connection"
messages when the request pool is overloaded. Since the checkout is
now done after passing ConcurrentLimiter.

This is technically less efficient, since the connection needs to be
checked in/out every time the middleware is left or entered respectively.
But I don't think the nanoseconds we might lose on redirects
to the same host are worth the complexity.

lib/pleroma/http/adapter_helper/hackney.ex

@@ -23,7 +23,4 @@ defmodule Pleroma.HTTP.AdapterHelper.Hackney do
   end
 
   defp add_scheme_opts(opts, _), do: opts
-
-  @spec get_conn(URI.t(), keyword()) :: {:ok, keyword()}
-  def get_conn(_uri, opts), do: {:ok, opts}
 end