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.