Integrate all (dummy) subsystems with the Overseer (#1374)

* overseer: introduce a utility typemap

* it's ugly but it compiles

* move DummySubsystem to subsystem crate

* fix tests fallout

* use a struct for all subsystems

* more tests fallout

* add missing pov_distribution subsystem

* remove unused imports and bounds

* fix minimal-example

polkadot/node/subsystem/src/lib.rs

@@ -148,3 +148,21 @@ pub trait Subsystem<C: SubsystemContext> {
 	/// Start this `Subsystem` and return `SpawnedSubsystem`.
 	fn start(self, ctx: C) -> SpawnedSubsystem;
 }
+
+/// A dummy subsystem that implements [`Subsystem`] for all
+/// types of messages. Used for tests or as a placeholder.
+pub struct DummySubsystem;
+
+impl<C: SubsystemContext> Subsystem<C> for DummySubsystem {
+	fn start(self, mut ctx: C) -> SpawnedSubsystem {
+		SpawnedSubsystem(Box::pin(async move {
+			loop {
+				match ctx.recv().await {
+					Ok(FromOverseer::Signal(OverseerSignal::Conclude)) => return,
+					Err(_) => return,
+					_ => continue,
+				}
+			}
+		}))
+	}
+}