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/service/src/lib.rs

@@ -77,7 +77,7 @@ pub use sc_chain_spec::{
 
 pub use sc_consensus::ImportQueue;
 pub use sc_executor::NativeExecutionDispatch;
-pub use sc_network_sync::warp::WarpSyncParams;
+pub use sc_network_sync::WarpSyncParams;
 #[doc(hidden)]
 pub use sc_network_transactions::config::{TransactionImport, TransactionImportFuture};
 pub use sc_rpc::{