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/statement/src/lib.rs

@@ -39,10 +39,8 @@ use sc_network::{
 	utils::{interval, LruHashSet},
 	NetworkEventStream, NetworkNotification, NetworkPeers,
 };
-use sc_network_common::{
-	role::ObservedRole,
-	sync::{SyncEvent, SyncEventStream},
-};
+use sc_network_common::role::ObservedRole;
+use sc_network_sync::{SyncEvent, SyncEventStream};
 use sp_statement_store::{
 	Hash, NetworkPriority, Statement, StatementSource, StatementStore, SubmitResult,
 };