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>

polkadot/node/service/src/lib.rs

@@ -734,7 +734,7 @@ pub fn new_full<OverseerGenerator: OverseerGen>(
 	}: NewFullParams<OverseerGenerator>,
 ) -> Result<NewFull, Error> {
 	use polkadot_node_network_protocol::request_response::IncomingRequest;
-	use sc_network_sync::warp::WarpSyncParams;
+	use sc_network_sync::WarpSyncParams;
 
 	let is_offchain_indexing_enabled = config.offchain_worker.indexing_enabled;
 	let role = config.role.clone();