Use async/await instead of manual polling of `NetworkWorker` (#13219)

* Convert `NetworkWorker::poll()` into async `next_action()`

* Use `NetworkWorker::next_action` instead of `poll` in `sc-network-test`

* Revert "Use `NetworkWorker::next_action` instead of `poll` in `sc-network-test`"

This reverts commit 4b5d851ec864f78f9d083a18a618fbe117c896d2.

* Fix `sc-network-test` to poll `NetworkWorker::next_action`

* Fix `sc_network::service` tests to poll `NetworkWorker::next_action`

* Fix docs

* kick CI

* Factor out `next_worker_message()` & `next_swarm_event()`

* Error handling: replace `futures::pending!()` with `expect()`

* Simplify stream polling in `select!`

* Replace `NetworkWorker::next_action()` with `run()`

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* minor: comment

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* Print debug log when network future is shut down

* Evaluate `NetworkWorker::run()` future once before the loop

* Fix client code to match new `NetworkService` interfaces

* Make clippy happy

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* Revert "Apply suggestions from code review"

This reverts commit 9fa646d0ed613e5f8623d3d37d1d59ec0a535850.

* Make `NetworkWorker::run()` consume `self`

* Terminate system RPC future if RPC rx stream has terminated.

* Rewrite with let-else

* Fix comments

* Get `best_seen_block` and call `on_block_finalized` via `ChainSync` instead of `NetworkService`

* rustfmt

* make clippy happy

* Tests: schedule wake if `next_action()` returned true

* minor: comment

* minor: fix `NetworkWorker` rustdoc

* minor: amend the rustdoc

* Fix bug that caused `on_demand_beefy_justification_sync` test to hang

* rustfmt

* Apply review suggestions

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Co-authored-by: Bastian Köcher <git@kchr.de>