Only fetch one collation at a time per relay parent (#3333)

* Only fetch one collation at a time per relay parent

Before a validator would fetch all collations that were advertised to
him. This pr changes the behavior to always just fetch one collation at
a time. If fetching fails, the validator will start fetching one of the
other collations.

* Use enum to be more explicit

* Review comments

polkadot/node/network/protocol/src/request_response/mod.rs

@@ -44,7 +44,7 @@ pub use sc_network::config::RequestResponseConfig;
 
 /// All requests that can be sent to the network bridge.
 pub mod request;
-pub use request::{IncomingRequest, OutgoingRequest, Requests, Recipient, OutgoingResult};
+pub use request::{IncomingRequest, OutgoingRequest, Requests, Recipient, OutgoingResult, ResponseSender};
 
 ///// Multiplexer for incoming requests.
 // pub mod multiplexer;

polkadot/node/network/protocol/src/request_response/request.rs

@@ -29,6 +29,9 @@ use crate::UnifiedReputationChange;
 
 use super::{v1, Protocol};
 
+/// Used by the network to send us a response to a request.
+pub type ResponseSender = oneshot::Sender<Result<Vec<u8>, network::RequestFailure>>;
+
 /// Common properties of any `Request`.
 pub trait IsRequest {
 	/// Each request has a corresponding `Response`.
@@ -109,7 +112,7 @@ pub struct OutgoingRequest<Req> {
 	/// The actual request to send over the wire.
 	pub payload: Req,
 	/// Sender which is used by networking to get us back a response.
-	pub pending_response: oneshot::Sender<Result<Vec<u8>, network::RequestFailure>>,
+	pub pending_response: ResponseSender,
 }
 
 /// Any error that can occur when sending a request.