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/state_request_handler.rs

@@ -17,7 +17,10 @@
 //! Helper for handling (i.e. answering) state requests from a remote peer via the
 //! `crate::request_responses::RequestResponsesBehaviour`.
 
-use crate::schema::v1::{KeyValueStateEntry, StateEntry, StateRequest, StateResponse};
+use crate::{
+	schema::v1::{KeyValueStateEntry, StateEntry, StateRequest, StateResponse},
+	LOG_TARGET,
+};
 
 use codec::{Decode, Encode};
 use futures::{channel::oneshot, stream::StreamExt};
@@ -39,7 +42,6 @@ use std::{
 	time::Duration,
 };
 
-const LOG_TARGET: &str = "sync";
 const MAX_RESPONSE_BYTES: usize = 2 * 1024 * 1024; // Actual reponse may be bigger.
 const MAX_NUMBER_OF_SAME_REQUESTS_PER_PEER: usize = 2;