Collator protocol subsystem (#1659)

* WIP

* The initial implementation of the collator side.

* Improve comments

* Multiple collation requests

* Add more tests and comments to validator side

* Add comments, remove dead code

* Apply suggestions from code review

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

* Fix build after suggested changes

* Also connect to the next validator group

* Remove a Future impl and move TimeoutExt to util

* Minor nits

* Fix build

* Change FetchCollations back to FetchCollation

* Try this

* Final fixes

* Fix build

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

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

@@ -18,13 +18,12 @@
 
 use polkadot_node_subsystem::messages::AllMessages;
 use polkadot_node_subsystem::{FromOverseer, SubsystemContext, SubsystemError, SubsystemResult};
+use polkadot_node_subsystem_util::TimeoutExt;
 
 use futures::channel::mpsc;
 use futures::poll;
 use futures::prelude::*;
-use futures_timer::Delay;
 use parking_lot::Mutex;
-use pin_project::pin_project;
 use sp_core::{testing::TaskExecutor, traits::SpawnNamed};
 
 use std::convert::Infallible;
@@ -284,45 +283,3 @@ pub fn subsystem_test_harness<M, OverseerFactory, Overseer, TestFactory, Test>(
 			.expect("test timed out instead of completing")
 	});
 }
-
-/// A future that wraps another future with a `Delay` allowing for time-limited futures.
-#[pin_project]
-pub struct Timeout<F: Future> {
-	#[pin]
-	future: F,
-	#[pin]
-	delay: Delay,
-}
-
-/// Extends `Future` to allow time-limited futures.
-pub trait TimeoutExt: Future {
-	fn timeout(self, duration: Duration) -> Timeout<Self>
-	where
-		Self: Sized,
-	{
-		Timeout {
-			future: self,
-			delay: Delay::new(duration),
-		}
-	}
-}
-
-impl<F: Future> TimeoutExt for F {}
-
-impl<F: Future> Future for Timeout<F> {
-	type Output = Option<F::Output>;
-
-	fn poll(self: Pin<&mut Self>, ctx: &mut Context) -> Poll<Self::Output> {
-		let this = self.project();
-
-		if this.delay.poll(ctx).is_ready() {
-			return Poll::Ready(None);
-		}
-
-		if let Poll::Ready(output) = this.future.poll(ctx) {
-			return Poll::Ready(Some(output));
-		}
-
-		Poll::Pending
-	}
-}