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/subsystem-types/src/messages.rs

@@ -232,7 +232,10 @@ pub enum DisputeCoordinatorMessage {
 	/// These disputes are either unconcluded or recently concluded.
 	ActiveDisputes(oneshot::Sender<Vec<(SessionIndex, CandidateHash)>>),
 	/// Get candidate votes for a candidate.
-	QueryCandidateVotes(SessionIndex, CandidateHash, oneshot::Sender<Option<CandidateVotes>>),
+	QueryCandidateVotes(
+		Vec<(SessionIndex, CandidateHash)>,
+		oneshot::Sender<Vec<(SessionIndex, CandidateHash, CandidateVotes)>>,
+	),
 	/// Sign and issue local dispute votes. A value of `true` indicates validity, and `false` invalidity.
 	IssueLocalStatement(SessionIndex, CandidateHash, CandidateReceipt, bool),
 	/// Determine the highest undisputed block within the given chain, based on where candidates