https://github.com/paritytech/polkadot-sdk/pull/3166 made private
functions used in `spawn_tasks()` public but forgot to add them in
exported functions of the crate.

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>