Extract syncing protocol from sc-network (#12828)

* 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.

* Move stuff to SyncingEngine

* Move `ChainSync` instanation to `SyncingEngine`

Some of the tests have to be rewritten

* Move peer hashmap to `SyncingEngine`

* Let `SyncingEngine` to implement `ChainSyncInterface`

* Introduce `SyncStatusProvider`

* Move `sync_peer_(connected|disconnected)` to `SyncingEngine`

* Implement `SyncEventStream`

Remove `SyncConnected`/`SyncDisconnected` events from
`NetworkEvenStream` and provide those events through
`ChainSyncInterface` instead.

Modify BEEFY/GRANDPA/transactions protocol and `NetworkGossip` to take
`SyncEventStream` object which they listen to for incoming sync peer
events.

* Introduce `ChainSyncInterface`

This interface provides a set of miscellaneous functions that other
subsystems can use to query, for example, the syncing status.

* Move event stream polling to `SyncingEngine`

Subscribe to `NetworkStreamEvent` and poll the incoming notifications
and substream events from `SyncingEngine`.

The code needs refactoring.

* Make `SyncingEngine` into an asynchronous runner

This commits removes the last hard dependency of syncing from
`sc-network` meaning the protocol now lives completely outside of
`sc-network`, ignoring the hardcoded peerset entry which will be
addressed in the future.

Code needs a lot of refactoring.

* Fix warnings

* Code refactoring

* Use `SyncingService` for BEEFY

* Use `SyncingService` for GRANDPA

* Remove call delegation from `NetworkService`

* Remove `ChainSyncService`

* Remove `ChainSync` service tests

They were written for the sole purpose of verifying that `NetworWorker`
continues to function while the calls are being dispatched to
`ChainSync`.

* Refactor code

* Refactor code

* Update client/finality-grandpa/src/communication/tests.rs

Co-authored-by: Anton <anton.kalyaev@gmail.com>

* Fix warnings

* Apply review comments

* Fix docs

* Fix test

* cargo-fmt

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

Co-authored-by: Anton <anton.kalyaev@gmail.com>

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

Co-authored-by: Anton <anton.kalyaev@gmail.com>

* Add missing docs

* Refactor code

---------

Co-authored-by: Anton <anton.kalyaev@gmail.com>

substrate/client/network/sync/src/service/mock.rs

@@ -23,7 +23,10 @@ use sc_network_common::{
 	config::MultiaddrWithPeerId,
 	protocol::ProtocolName,
 	request_responses::{IfDisconnected, RequestFailure},
-	service::{NetworkPeers, NetworkRequest, NetworkSyncForkRequest},
+	service::{
+		NetworkNotification, NetworkPeers, NetworkRequest, NetworkSyncForkRequest,
+		NotificationSender, NotificationSenderError,
+	},
 };
 use sc_peerset::ReputationChange;
 use sp_runtime::traits::{Block as BlockT, NumberFor};
@@ -125,4 +128,14 @@ mockall::mock! {
 			connect: IfDisconnected,
 		);
 	}
+
+	impl NetworkNotification for Network {
+		fn write_notification(&self, target: PeerId, protocol: ProtocolName, message: Vec<u8>);
+		fn notification_sender(
+			&self,
+			target: PeerId,
+			protocol: ProtocolName,
+		) -> Result<Box<dyn NotificationSender>, NotificationSenderError>;
+		fn set_notification_handshake(&self, protocol: ProtocolName, handshake: Vec<u8>);
+	}
 }