Dispute Coordinator: Batch queries (#3459)

* disputes: Allow batch queries in dispute-coordinator

This commit moves to batch queries when responding to QueryCandidateVotes
messages. This simplifies the code in the provisioner and dispute-coordinator
by no longer requiring to make use of a FuturesOrdered when awaiting multiple
quries. Instead, the provisioner need only request the batch itself.

* node/approval-voting: Address Feedback to fail on query element missing.

* Address feedback

* Fix implementer's guide

polkadot/node/network/dispute-distribution/src/sender/mod.rs

@@ -352,11 +352,12 @@ async fn get_candidate_votes<Context: SubsystemContext>(
 	let (tx, rx) = oneshot::channel();
 	ctx.send_message(AllMessages::DisputeCoordinator(
 		DisputeCoordinatorMessage::QueryCandidateVotes(
-			session_index,
-			candidate_hash,
+			vec![(session_index, candidate_hash)],
 			tx
 		)
 	))
 	.await;
-	rx.await.map_err(|_| NonFatal::AskCandidateVotesCanceled)
+	rx.await
+		.map(|v| v.get(0).map(|inner| inner.to_owned().2))
+		.map_err(|_| NonFatal::AskCandidateVotesCanceled)
 }