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/network/test/src/fuzz.rs

@@ -20,7 +20,6 @@
 //! and `PeerStore` to discover possible inconsistencies in peer management.
 
 use futures::prelude::*;
-use libp2p::PeerId;
 use rand::{
 	distributions::{Distribution, Uniform, WeightedIndex},
 	seq::IteratorRandom,
@@ -28,10 +27,13 @@ use rand::{
 use sc_network::{
 	peer_store::{PeerStore, PeerStoreProvider},
 	protocol_controller::{IncomingIndex, Message, ProtoSetConfig, ProtocolController, SetId},
-	ReputationChange,
+	PeerId, ReputationChange,
 };
 use sc_utils::mpsc::tracing_unbounded;
-use std::collections::{HashMap, HashSet};
+use std::{
+	collections::{HashMap, HashSet},
+	sync::Arc,
+};
 
 /// Peer events as observed by `Notifications` / fuzz test.
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
@@ -141,7 +143,7 @@ async fn test_once() {
 		.collect();
 
 	let peer_store = PeerStore::new(bootnodes);
-	let mut peer_store_handle = peer_store.handle();
+	let peer_store_handle = peer_store.handle();
 
 	let (to_notifications, mut from_controller) =
 		tracing_unbounded("test_to_notifications", 10_000);
@@ -163,7 +165,7 @@ async fn test_once() {
 			reserved_only: Uniform::new_inclusive(0, 10).sample(&mut rng) == 0,
 		},
 		to_notifications,
-		Box::new(peer_store_handle.clone()),
+		Arc::new(peer_store_handle.clone()),
 	);
 
 	tokio::spawn(peer_store.run());
@@ -319,14 +321,15 @@ async fn test_once() {
 				1 => {
 					let new_id = PeerId::random();
 					known_nodes.insert(new_id, State::Disconnected);
-					peer_store_handle.add_known_peer(new_id);
+					peer_store_handle.add_known_peer(new_id.into());
 				},
 
 				// If we generate 2, adjust a random reputation.
 				2 =>
 					if let Some(id) = known_nodes.keys().choose(&mut rng) {
 						let val = Uniform::new_inclusive(i32::MIN, i32::MAX).sample(&mut rng);
-						peer_store_handle.report_peer(*id, ReputationChange::new(val, ""));
+						let peer: sc_network_types::PeerId = id.into();
+						peer_store_handle.report_peer(peer, ReputationChange::new(val, ""));
 					},
 
 				// If we generate 3, disconnect from a random node.
@@ -414,5 +417,6 @@ async fn test_once() {
 			}
 		}
 	})
-	.await;
+	.await
+	.unwrap();
 }