Split NetworkBridge and break cycles with Unbounded (#2736)

* overseer: pass messages directly between subsystems

* test that message is held on to

* Update node/overseer/src/lib.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* give every subsystem an unbounded sender too

* remove metered_channel::name

1. we don't provide good names
2. these names are never used anywhere

* unused mut

* remove unnecessary &mut

* subsystem unbounded_send

* remove unused MaybeTimer

We have channel size metrics that serve the same purpose better now and the implementation of message timing was pretty ugly.

* remove comment

* split up senders and receivers

* update metrics

* fix tests

* fix test subsystem context

* use SubsystemSender in jobs system now

* refactor of awful jobs code

* expose public `run` on JobSubsystem

* update candidate backing to new jobs & use unbounded

* bitfield signing

* candidate-selection

* provisioner

* approval voting: send unbounded for assignment/approvals

* async not needed

* begin bridge split

* split up network tasks into background worker

* port over network bridge

* Update node/network/bridge/src/lib.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* rename ValidationWorkerNotifications

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>
Co-authored-by: Andronik Ordian <write@reusable.software>

polkadot/node/subsystem-test-helpers/src/lib.rs

@@ -162,6 +162,15 @@ pub struct TestSubsystemSender {
 	tx: mpsc::UnboundedSender<AllMessages>,
 }
 
+/// Construct a sender/receiver pair.
+pub fn sender_receiver() -> (TestSubsystemSender, mpsc::UnboundedReceiver<AllMessages>) {
+	let (tx, rx) = mpsc::unbounded();
+	(
+		TestSubsystemSender { tx },
+		rx,
+	)
+}
+
 #[async_trait::async_trait]
 impl SubsystemSender for TestSubsystemSender {
 	async fn send_message(&mut self, msg: AllMessages) {