Currently, an mpstream is initialized with the Lua error handler in
netbox_prepare_request, which is used by all encoding methods, including
netbox_encode_auth. The latter will be moved to C, along with iproto
request handlers, where we will have to use a different error handler.
Let's create an mpstream in netbox_encode_method and netbox_encode_auth
instead. For now, they do the same, but once we move the code to C they
will use different error handlers.

Part of #6241