Fix polkadot parachains not producing blocks until next session (#4269)
... a few sessions too late :(, this already happened on polkadot, so as of now there are no known relay-chains without async backing enabled in runtime, but let's fix it in case someone else wants to repeat our steps. Fixes: https://github.com/paritytech/polkadot-sdk/issues/4226 --------- Signed-off-by: Alexandru Gheorghe <[email protected]>