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/mixnet/src/protocol.rs

@@ -19,9 +19,12 @@
 use super::config::Config;
 use mixnet::core::PACKET_SIZE;
 use sc_network::{
-	config::{NonDefaultSetConfig, NonReservedPeerMode, SetConfig},
-	NotificationService, ProtocolName,
+	config::{NonReservedPeerMode, SetConfig},
+	peer_store::PeerStoreProvider,
+	service::NotificationMetrics,
+	NetworkBackend, NotificationService, ProtocolName,
 };
+use sp_runtime::traits::Block as BlockT;
 
 /// Returns the protocol name to use for the mixnet controlled by the given chain.
 pub fn protocol_name(genesis_hash: &[u8], fork_id: Option<&str>) -> ProtocolName {
@@ -34,26 +37,37 @@ pub fn protocol_name(genesis_hash: &[u8], fork_id: Option<&str>) -> ProtocolName
 }
 
 /// Returns the peers set configuration for the mixnet protocol.
-pub fn peers_set_config(
+pub fn peers_set_config<Block: BlockT, Network: NetworkBackend<Block, <Block as BlockT>::Hash>>(
 	name: ProtocolName,
 	config: &Config,
-) -> (NonDefaultSetConfig, Box<dyn NotificationService>) {
-	let (mut set_config, service) = NonDefaultSetConfig::new(
-		name,
-		Vec::new(),
-		PACKET_SIZE as u64,
-		None,
+	metrics: NotificationMetrics,
+	peerstore_handle: std::sync::Arc<dyn PeerStoreProvider>,
+) -> (Network::NotificationProtocolConfig, Box<dyn NotificationService>) {
+	let set_config = if config.substrate.num_gateway_slots != 0 {
+		// out_peers is always 0; we are only interested in connecting to mixnodes, which we do by
+		// setting them as reserved nodes
+		SetConfig {
+			in_peers: config.substrate.num_gateway_slots,
+			out_peers: 0,
+			reserved_nodes: Vec::new(),
+			non_reserved_mode: NonReservedPeerMode::Accept,
+		}
+	} else {
 		SetConfig {
 			in_peers: 0,
 			out_peers: 0,
 			reserved_nodes: Vec::new(),
 			non_reserved_mode: NonReservedPeerMode::Deny,
-		},
-	);
-	if config.substrate.num_gateway_slots != 0 {
-		// out_peers is always 0; we are only interested in connecting to mixnodes, which we do by
-		// setting them as reserved nodes
-		set_config.allow_non_reserved(config.substrate.num_gateway_slots, 0);
-	}
-	(set_config, service)
+		}
+	};
+
+	Network::notification_config(
+		name,
+		Vec::new(),
+		PACKET_SIZE as u64,
+		None,
+		set_config,
+		metrics,
+		peerstore_handle,
+	)
 }