Move requests-responses and polling from ChainSync to SyncingEngine (#1650)

Move request-response handling from `ChainSync` to `SyncingEngine` as
part of [Sync
2.0](https://github.com/paritytech/polkadot-sdk/issues/534) refactoring
aimed at making `ChainSync` a pure state machine.

Resolves https://github.com/paritytech/polkadot-sdk/issues/502.

---------

Co-authored-by: Aaro Altonen <48052676+altonen@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>

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

@@ -20,7 +20,7 @@
 
 use crate::block_relay_protocol::{BlockDownloader as BlockDownloaderT, BlockResponseError};
 
-use futures::{channel::oneshot, task::Poll};
+use futures::channel::oneshot;
 use libp2p::PeerId;
 use sc_network::RequestFailure;
 use sc_network_common::sync::{
@@ -39,7 +39,6 @@ mockall::mock! {
 		fn num_sync_requests(&self) -> usize;
 		fn num_downloaded_blocks(&self) -> usize;
 		fn num_peers(&self) -> usize;
-		fn num_active_peers(&self) -> usize;
 		fn new_peer(
 			&mut self,
 			who: PeerId,
@@ -81,15 +80,6 @@ mockall::mock! {
 		);
 		fn peer_disconnected(&mut self, who: &PeerId);
 		fn metrics(&self) -> Metrics;
-		fn poll<'a>(
-			&mut self,
-			cx: &mut std::task::Context<'a>,
-		) -> Poll<()>;
-		fn send_block_request(
-			&mut self,
-			who: PeerId,
-			request: BlockRequest<Block>,
-		);
 	}
 }