Move import queue out of sc-network (#12764)

* Move import queue out of `sc-network`

Add supplementary asynchronous API for the import queue which means
it can be run as an independent task and communicated with through
the `ImportQueueService`.

This commit removes removes block and justification imports from
`sc-network` and provides `ChainSync` with a handle to import queue so
it can import blocks and justifications. Polling of the import queue is
moved complete out of `sc-network` and `sc_consensus::Link` is
implemented for `ChainSyncInterfaceHandled` so the import queue
can still influence the syncing process.

* Fix tests

* Apply review comments

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update client/network/sync/src/lib.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/client/network/sync/src/tests.rs

@@ -37,6 +37,7 @@ use substrate_test_runtime_client::{TestClientBuilder, TestClientBuilderExt as _
 // poll `ChainSync` and verify that a new sync fork request has been registered
 #[tokio::test]
 async fn delegate_to_chainsync() {
+	let import_queue = Box::new(sc_consensus::import_queue::mock::MockImportQueueHandle::new());
 	let (_chain_sync_network_provider, chain_sync_network_handle) = NetworkServiceProvider::new();
 	let (mut chain_sync, chain_sync_service, _) = ChainSync::new(
 		sc_network_common::sync::SyncMode::Full,
@@ -47,7 +48,9 @@ async fn delegate_to_chainsync() {
 		Box::new(DefaultBlockAnnounceValidator),
 		1u32,
 		None,
+		None,
 		chain_sync_network_handle,
+		import_queue,
 		ProtocolName::from("block-request"),
 		ProtocolName::from("state-request"),
 		None,