Upgrade tokio to 1.22.0 and replace async-std with tokio (#12646)

* Replace deprecated libp2p feature specs with correct ones

* Bump tokio to 1.21.2

* Replace async-std libp2p primitives with tokio ones

* minor: rustfmt

* Fix TestNet to run initialization in the tokio context

* Convert telemetry test from async-std to tokio

* Convert notifications tests from async-std to tokio

* Convert chain sync tests from async-std to tokio

* Ditch async-std completely

* Make executor mandatory

* Bump tokio to 1.22.0

* minor: rustfmt

* Explicitly use tokio runtime in tests

* Move more tests to explicit tokio runtime

* Explicitly set multithreaded runtime in tokio test

* minor: rustfmt

* minor: fix comment

* Replace async-std with tokio in MMR tests