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.
Dmitry Markin