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/bin/node/cli/src/service.rs

@@ -30,7 +30,7 @@ use node_primitives::Block;
 use sc_client_api::{Backend, BlockBackend};
 use sc_consensus_babe::{self, SlotProportion};
 use sc_network::{event::Event, NetworkEventStream, NetworkService};
-use sc_network_sync::{warp::WarpSyncParams, SyncingService};
+use sc_network_sync::{strategy::warp::WarpSyncParams, SyncingService};
 use sc_service::{config::Configuration, error::Error as ServiceError, RpcHandlers, TaskManager};
 use sc_statement_store::Store as StatementStore;
 use sc_telemetry::{Telemetry, TelemetryWorker};