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/lib.rs

@@ -19,10 +19,10 @@
 //! Blockchain syncing implementation in Substrate.
 
 pub use service::syncing_service::SyncingService;
+pub use strategy::warp::{WarpSyncParams, WarpSyncPhase, WarpSyncProgress};
 pub use types::{SyncEvent, SyncEventStream, SyncState, SyncStatus, SyncStatusProvider};
 
 mod block_announce_validator;
-mod chain_sync;
 mod extra_requests;
 mod futures_stream;
 mod pending_responses;
@@ -36,7 +36,9 @@ pub mod blocks;
 pub mod engine;
 pub mod mock;
 pub mod service;
-pub mod state;
 pub mod state_request_handler;
-pub mod warp;
+pub mod strategy;
 pub mod warp_request_handler;
+
+/// Log target for this crate.
+const LOG_TARGET: &str = "sync";