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/pending_responses.rs

@@ -19,7 +19,7 @@
 //! [`PendingResponses`] is responsible for keeping track of pending responses and
 //! polling them. [`Stream`] implemented by [`PendingResponses`] never terminates.
 
-use crate::types::PeerRequest;
+use crate::{types::PeerRequest, LOG_TARGET};
 use futures::{
 	channel::oneshot,
 	future::BoxFuture,
@@ -33,9 +33,6 @@ use sp_runtime::traits::Block as BlockT;
 use std::task::{Context, Poll, Waker};
 use tokio_stream::StreamMap;
 
-/// Log target for this file.
-const LOG_TARGET: &'static str = "sync";
-
 /// Response result.
 type ResponseResult = Result<Result<(Vec<u8>, ProtocolName), RequestFailure>, oneshot::Canceled>;