Integrate litep2p into Polkadot SDK (#2944)

[litep2p](https://github.com/altonen/litep2p) is a libp2p-compatible P2P
networking library. It supports all of the features of `rust-libp2p`
that are currently being utilized by Polkadot SDK.

Compared to `rust-libp2p`, `litep2p` has a quite different architecture
which is why the new `litep2p` network backend is only able to use a
little of the existing code in `sc-network`. The design has been mainly
influenced by how we'd wish to structure our networking-related code in
Polkadot SDK: independent higher-levels protocols directly communicating
with the network over links that support bidirectional backpressure. A
good example would be `NotificationHandle`/`RequestResponseHandle`
abstractions which allow, e.g., `SyncingEngine` to directly communicate
with peers to announce/request blocks.

I've tried running `polkadot --network-backend litep2p` with a few
different peer configurations and there is a noticeable reduction in
networking CPU usage. For high load (`--out-peers 200`), networking CPU
usage goes down from ~110% to ~30% (80 pp) and for normal load
(`--out-peers 40`), the usage goes down from ~55% to ~18% (37 pp).

These should not be taken as final numbers because:

a) there are still some low-hanging optimization fruits, such as
enabling [receive window
auto-tuning](https://github.com/libp2p/rust-yamux/pull/176), integrating
`Peerset` more closely with `litep2p` or improving memory usage of the
WebSocket transport
b) fixing bugs/instabilities that incorrectly cause `litep2p` to do less
work will increase the networking CPU usage
c) verification in a more diverse set of tests/conditions is needed

Nevertheless, these numbers should give an early estimate for CPU usage
of the new networking backend.

This PR consists of three separate changes:
* introduce a generic `PeerId` (wrapper around `Multihash`) so that we
don't have use `NetworkService::PeerId` in every part of the code that
uses a `PeerId`
* introduce `NetworkBackend` trait, implement it for the libp2p network
stack and make Polkadot SDK generic over `NetworkBackend`
  * implement `NetworkBackend` for litep2p

The new library should be considered experimental which is why
`rust-libp2p` will remain as the default option for the time being. This
PR currently depends on the master branch of `litep2p` but I'll cut a
new release for the library once all review comments have been
addresses.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
Co-authored-by: Alexandru Vasile <alexandru.vasile@parity.io>

substrate/client/service/src/lib.rs

@@ -42,9 +42,11 @@ use jsonrpsee::RpcModule;
 use log::{debug, error, warn};
 use sc_client_api::{blockchain::HeaderBackend, BlockBackend, BlockchainEvents, ProofProvider};
 use sc_network::{
-	config::MultiaddrWithPeerId, NetworkBlock, NetworkPeers, NetworkStateInfo, PeerId,
+	config::MultiaddrWithPeerId, service::traits::NetworkService, NetworkBackend, NetworkBlock,
+	NetworkPeers, NetworkStateInfo,
 };
 use sc_network_sync::SyncingService;
+use sc_network_types::PeerId;
 use sc_utils::mpsc::TracingUnboundedReceiver;
 use sp_blockchain::HeaderMetadata;
 use sp_consensus::SyncOracle;
@@ -157,8 +159,9 @@ async fn build_network_future<
 		+ Sync
 		+ 'static,
 	H: sc_network_common::ExHashT,
+	N: NetworkBackend<B, <B as BlockT>::Hash>,
 >(
-	network: sc_network::NetworkWorker<B, H>,
+	network: N,
 	client: Arc<C>,
 	sync_service: Arc<SyncingService<B>>,
 	announce_imported_blocks: bool,
@@ -225,7 +228,7 @@ pub async fn build_system_rpc_future<
 	H: sc_network_common::ExHashT,
 >(
 	role: Role,
-	network_service: Arc<sc_network::NetworkService<B, H>>,
+	network_service: Arc<dyn NetworkService>,
 	sync_service: Arc<SyncingService<B>>,
 	client: Arc<C>,
 	mut rpc_rx: TracingUnboundedReceiver<sc_rpc::system::Request<B>>,
@@ -310,14 +313,12 @@ pub async fn build_system_rpc_future<
 				};
 			},
 			sc_rpc::system::Request::NetworkReservedPeers(sender) => {
-				let reserved_peers = network_service.reserved_peers().await;
-				if let Ok(reserved_peers) = reserved_peers {
-					let reserved_peers =
-						reserved_peers.iter().map(|peer_id| peer_id.to_base58()).collect();
-					let _ = sender.send(reserved_peers);
-				} else {
-					break
-				}
+				let Ok(reserved_peers) = network_service.reserved_peers().await else {
+					break;
+				};
+
+				let _ =
+					sender.send(reserved_peers.iter().map(|peer_id| peer_id.to_base58()).collect());
 			},
 			sc_rpc::system::Request::NodeRoles(sender) => {
 				use sc_rpc::system::NodeRole;