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.