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/warp_request_handler.rs

@@ -20,7 +20,10 @@ use codec::Decode;
 use futures::{channel::oneshot, stream::StreamExt};
 use log::debug;
 
-use crate::warp::{EncodedProof, WarpProofRequest, WarpSyncProvider};
+use crate::{
+	strategy::warp::{EncodedProof, WarpProofRequest, WarpSyncProvider},
+	LOG_TARGET,
+};
 use sc_network::{
 	config::ProtocolId,
 	request_responses::{
@@ -120,10 +123,10 @@ impl<TBlock: BlockT> RequestHandler<TBlock> {
 
 			match self.handle_request(payload, pending_response) {
 				Ok(()) => {
-					debug!(target: "sync", "Handled grandpa warp sync request from {}.", peer)
+					debug!(target: LOG_TARGET, "Handled grandpa warp sync request from {}.", peer)
 				},
 				Err(e) => debug!(
-					target: "sync",
+					target: LOG_TARGET,
 					"Failed to handle grandpa warp sync request from {}: {}",
 					peer, e,
 				),