Move syncing code from sc-network-common to sc-network-sync (#1912)

This PR moves syncing-related code from `sc-network-common` to
`sc-network-sync`.

Unfortunately, some parts are tightly integrated with networking, so
they were left in `sc-network-common` for now:

1. `SyncMode` in `common/src/sync.rs` (used in `NetworkConfiguration`).
2. `BlockAnnouncesHandshake`, `BlockRequest`, `BlockResponse`, etc. in
`common/src/sync/message.rs` (used in `src/protocol.rs` and
`src/protocol/message.rs`).

More substantial refactoring is needed to decouple syncing and
networking completely, including getting rid of the hardcoded sync
protocol.

## Release notes

Move syncing-related code from `sc-network-common` to `sc-network-sync`.
Delete `ChainSync` trait as it's never used (the only implementation is
accessed directly from `SyncingEngine` and exposes a lot of public
methods that are not part of the trait). Some new trait(s) for syncing
will likely be introduced as part of Sync 2.0 refactoring to represent
syncing strategies.

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

@@ -16,11 +16,13 @@
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
-use crate::{PeerSync, PeerSyncState};
+use crate::{
+	chain_sync::{PeerSync, PeerSyncState},
+	request_metrics::Metrics,
+};
 use fork_tree::ForkTree;
 use libp2p::PeerId;
 use log::{debug, trace, warn};
-use sc_network_common::sync::metrics::Metrics;
 use sp_blockchain::Error as ClientError;
 use sp_runtime::traits::{Block as BlockT, NumberFor, Zero};
 use std::{
@@ -343,7 +345,7 @@ impl<'a, B: BlockT> Matcher<'a, B> {
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use crate::PeerSync;
+	use crate::chain_sync::PeerSync;
 	use quickcheck::{Arbitrary, Gen, QuickCheck};
 	use sp_blockchain::Error as ClientError;
 	use sp_test_primitives::{Block, BlockNumber, Hash};