Extract warp sync strategy from ChainSync (#2467)

Extract `WarpSync` (and `StateSync` as part of warp sync) from
`ChainSync` as independent syncing strategy called by `SyncingEngine`.
Introduce `SyncingStrategy` enum as a proxy between `SyncingEngine` and
specific syncing strategies.

## Limitations
Gap sync is kept in `ChainSync` for now because it shares the same set
of peers as block syncing implementation in `ChainSync`. Extraction of a
common context responsible for peer management in syncing strategies
able to run in parallel is planned for a follow-up PR.

## Further improvements
A possibility of conversion of `SyncingStartegy` into a trait should be
evaluated. The main stopper for this is that different strategies need
to communicate different actions to `SyncingEngine` and respond to
different events / provide different APIs (e.g., requesting
justifications is only possible via `ChainSync` and not through
`WarpSync`; `SendWarpProofRequest` action is only relevant to
`WarpSync`, etc.)

---------

Co-authored-by: Aaro Altonen <48052676+altonen@users.noreply.github.com>

substrate/client/network/sync/src/block_request_handler.rs

@@ -24,6 +24,7 @@ use crate::{
 		BlockResponse as BlockResponseSchema, BlockResponse, Direction,
 	},
 	service::network::NetworkServiceHandle,
+	LOG_TARGET,
 };
 
 use codec::{Decode, DecodeAll, Encode};
@@ -56,7 +57,6 @@ use std::{
 /// 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;