Add ClaimQueue wrapper (#3950)

Remove `fetch_next_scheduled_on_core` in favor of new wrapper and
methods for accessing it.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/collation-generation/src/lib.rs

@@ -45,13 +45,12 @@ use polkadot_node_subsystem::{
 use polkadot_node_subsystem_util::{
 	request_async_backing_params, request_availability_cores, request_para_backing_state,
 	request_persisted_validation_data, request_validation_code, request_validation_code_hash,
-	request_validators,
-	vstaging::{fetch_claim_queue, fetch_next_scheduled_on_core},
+	request_validators, vstaging::fetch_claim_queue,
 };
 use polkadot_primitives::{
 	collator_signature_payload, CandidateCommitments, CandidateDescriptor, CandidateReceipt,
 	CollatorPair, CoreIndex, CoreState, Hash, Id as ParaId, OccupiedCoreAssumption,
-	PersistedValidationData, ValidationCodeHash,
+	PersistedValidationData, ScheduledCore, ValidationCodeHash,
 };
 use sp_core::crypto::Pair;
 use std::sync::Arc;
@@ -245,11 +244,10 @@ async fn handle_new_activations<Context>(
 						// Use claim queue if available, or fallback to `next_up_on_available`
 						let res = match maybe_claim_queue {
 							Some(ref claim_queue) => {
-								// read what's in the claim queue for this core
-								fetch_next_scheduled_on_core(
-									claim_queue,
-									CoreIndex(core_idx as u32),
-								)
+								// read what's in the claim queue for this core at depth 0.
+								claim_queue
+									.get_claim_for(CoreIndex(core_idx as u32), 0)
+									.map(|para_id| ScheduledCore { para_id, collator: None })
 							},
 							None => {
 								// Runtime doesn't support claim queue runtime api. Fallback to