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>

polkadot/node/network/protocol/src/peer_set.rs

@@ -19,13 +19,14 @@
 use derive_more::Display;
 use polkadot_primitives::Hash;
 use sc_network::{
-	config::{NonDefaultSetConfig, SetConfig},
-	types::ProtocolName,
-	NotificationService,
+	config::SetConfig, peer_store::PeerStoreProvider, service::NotificationMetrics,
+	types::ProtocolName, NetworkBackend, NotificationService,
 };
+use sp_runtime::traits::Block;
 use std::{
 	collections::{hash_map::Entry, HashMap},
 	ops::{Index, IndexMut},
+	sync::Arc,
 };
 use strum::{EnumIter, IntoEnumIterator};
 
@@ -65,11 +66,13 @@ impl PeerSet {
 	///
 	/// Those should be used in the network configuration to register the protocols with the
 	/// network service.
-	pub fn get_info(
+	pub fn get_info<B: Block, N: NetworkBackend<B, <B as Block>::Hash>>(
 		self,
 		is_authority: IsAuthority,
 		peerset_protocol_names: &PeerSetProtocolNames,
-	) -> (NonDefaultSetConfig, (PeerSet, Box<dyn NotificationService>)) {
+		metrics: NotificationMetrics,
+		peer_store_handle: Arc<dyn PeerStoreProvider>,
+	) -> (N::NotificationProtocolConfig, (PeerSet, Box<dyn NotificationService>)) {
 		// Networking layer relies on `get_main_name()` being the main name of the protocol
 		// for peersets and connection management.
 		let protocol = peerset_protocol_names.get_main_name(self);
@@ -82,7 +85,7 @@ impl PeerSet {
 
 		match self {
 			PeerSet::Validation => {
-				let (config, notification_service) = NonDefaultSetConfig::new(
+				let (config, notification_service) = N::notification_config(
 					protocol,
 					fallback_names,
 					max_notification_size,
@@ -97,12 +100,14 @@ impl PeerSet {
 						reserved_nodes: Vec::new(),
 						non_reserved_mode: sc_network::config::NonReservedPeerMode::Accept,
 					},
+					metrics,
+					peer_store_handle,
 				);
 
 				(config, (PeerSet::Validation, notification_service))
 			},
 			PeerSet::Collation => {
-				let (config, notification_service) = NonDefaultSetConfig::new(
+				let (config, notification_service) = N::notification_config(
 					protocol,
 					fallback_names,
 					max_notification_size,
@@ -119,6 +124,8 @@ impl PeerSet {
 							sc_network::config::NonReservedPeerMode::Deny
 						},
 					},
+					metrics,
+					peer_store_handle,
 				);
 
 				(config, (PeerSet::Collation, notification_service))
@@ -207,12 +214,21 @@ impl<T> IndexMut<PeerSet> for PerPeerSet<T> {
 ///
 /// Should be used during network configuration (added to `NetworkConfiguration::extra_sets`)
 /// or shortly after startup to register the protocols with the network service.
-pub fn peer_sets_info(
+pub fn peer_sets_info<B: Block, N: NetworkBackend<B, <B as Block>::Hash>>(
 	is_authority: IsAuthority,
 	peerset_protocol_names: &PeerSetProtocolNames,
-) -> Vec<(NonDefaultSetConfig, (PeerSet, Box<dyn NotificationService>))> {
+	metrics: NotificationMetrics,
+	peer_store_handle: Arc<dyn PeerStoreProvider>,
+) -> Vec<(N::NotificationProtocolConfig, (PeerSet, Box<dyn NotificationService>))> {
 	PeerSet::iter()
-		.map(|s| s.get_info(is_authority, &peerset_protocol_names))
+		.map(|s| {
+			s.get_info::<B, N>(
+				is_authority,
+				&peerset_protocol_names,
+				metrics.clone(),
+				Arc::clone(&peer_store_handle),
+			)
+		})
 		.collect()
 }