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/block_request_handler.rs

@@ -24,7 +24,6 @@ use crate::{
 		BlockResponse as BlockResponseSchema, BlockResponse, Direction,
 	},
 	service::network::NetworkServiceHandle,
-	MAX_BLOCKS_IN_RESPONSE,
 };
 
 use codec::{Decode, DecodeAll, Encode};
@@ -54,6 +53,9 @@ use std::{
 	time::Duration,
 };
 
+/// Maximum blocks per response.
+pub(crate) const MAX_BLOCKS_IN_RESPONSE: usize = 128;
+
 const LOG_TARGET: &str = "sync";
 const MAX_BODY_BYTES: usize = 8 * 1024 * 1024;
 const MAX_NUMBER_OF_SAME_REQUESTS_PER_PEER: usize = 2;