Problems with new async-std

[svyatonik]

#107 bumps async-std to 1.6.0. This breaks PoA <-> Substrate relay - it stops waking up => new headers are not submitted.

[svyatonik]

So the reason seems to be migration of async-std to smol that has happened recently and the fact that current smol has some by-design problems with how we use timers.

The minimal (hanging) test would be:

let mut local_pool = futures::executor::LocalPool::new();
local_pool.run_until(async move {
    async_std::task::sleep(std::time::Duration::from_secs(1)).await;
});

So my suggestion is to wait until async-std will support our way of sleeping again.