Refactor sc_network::NetworkService::poll to use async/await

[tomaka]

Continuation of the discussion in #6903

We should basically re-do https://github.com/paritytech/substrate/pull/5704/files.

[tomaka]

Not difficult per se, but marked as "medium" because it's error-prone.

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.

[tomaka]

Issue still relevant and important.