Use async/await instead of manual polling of `NetworkWorker`

[dmitry-markin]

Resolves https://github.com/paritytech/substrate/issues/6919. Re-does https://github.com/paritytech/substrate/pull/5704.

I also tried rewriting sc-network-test to use async/await instead of polling, the first attempt is here: https://github.com/paritytech/substrate/commit/4b5d851ec864f78f9d083a18a618fbe117c896d2. It turned out that doing it without slowing down the tests is less trivial than I expected, so I think we should do it in a separate PR.

cumulus companion: https://github.com/paritytech/cumulus/pull/2127