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/src/protocol.rs

@@ -18,9 +18,9 @@
 
 use crate::{
 	config, error,
-	peer_store::{PeerStoreHandle, PeerStoreProvider},
+	peer_store::PeerStoreProvider,
 	protocol_controller::{self, SetId},
-	service::traits::Direction,
+	service::{metrics::NotificationMetrics, traits::Direction},
 	types::ProtocolName,
 };
 
@@ -36,14 +36,13 @@ use libp2p::{
 use log::warn;
 
 use codec::DecodeAll;
-use prometheus_endpoint::Registry;
 use sc_network_common::role::Roles;
 use sc_utils::mpsc::TracingUnboundedReceiver;
 use sp_runtime::traits::Block as BlockT;
 
-use std::{collections::HashSet, iter, task::Poll};
+use std::{collections::HashSet, iter, sync::Arc, task::Poll};
 
-use notifications::{metrics, Notifications, NotificationsOut};
+use notifications::{Notifications, NotificationsOut};
 
 pub(crate) use notifications::ProtocolHandle;
 
@@ -69,7 +68,7 @@ pub struct Protocol<B: BlockT> {
 	/// List of notifications protocols that have been registered.
 	notification_protocols: Vec<ProtocolName>,
 	/// Handle to `PeerStore`.
-	peer_store_handle: PeerStoreHandle,
+	peer_store_handle: Arc<dyn PeerStoreProvider>,
 	/// Streams for peers whose handshake couldn't be determined.
 	bad_handshake_streams: HashSet<PeerId>,
 	sync_handle: ProtocolHandle,
@@ -80,10 +79,10 @@ impl<B: BlockT> Protocol<B> {
 	/// Create a new instance.
 	pub(crate) fn new(
 		roles: Roles,
-		registry: &Option<Registry>,
+		notification_metrics: NotificationMetrics,
 		notification_protocols: Vec<config::NonDefaultSetConfig>,
 		block_announces_protocol: config::NonDefaultSetConfig,
-		peer_store_handle: PeerStoreHandle,
+		peer_store_handle: Arc<dyn PeerStoreProvider>,
 		protocol_controller_handles: Vec<protocol_controller::ProtocolHandle>,
 		from_protocol_controllers: TracingUnboundedReceiver<protocol_controller::Message>,
 	) -> error::Result<(Self, Vec<ProtocolHandle>)> {
@@ -122,16 +121,15 @@ impl<B: BlockT> Protocol<B> {
 			}))
 			.unzip();
 
-			let metrics = registry.as_ref().and_then(|registry| metrics::register(&registry).ok());
 			handles.iter_mut().for_each(|handle| {
-				handle.set_metrics(metrics.clone());
+				handle.set_metrics(notification_metrics.clone());
 			});
 
 			(
 				Notifications::new(
 					protocol_controller_handles,
 					from_protocol_controllers,
-					metrics,
+					notification_metrics,
 					protocol_configs.into_iter(),
 				),
 				installed_protocols,
@@ -179,7 +177,7 @@ impl<B: BlockT> Protocol<B> {
 	fn role_available(&self, peer_id: &PeerId, handshake: &Vec<u8>) -> bool {
 		match Roles::decode_all(&mut &handshake[..]) {
 			Ok(_) => true,
-			Err(_) => self.peer_store_handle.peer_role(&peer_id).is_some(),
+			Err(_) => self.peer_store_handle.peer_role(&((*peer_id).into())).is_some(),
 		}
 	}
 }