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/request_responses.rs

@@ -36,6 +36,7 @@
 
 use crate::{
 	peer_store::{PeerStoreProvider, BANNED_THRESHOLD},
+	service::traits::RequestResponseConfig as RequestResponseConfigT,
 	types::ProtocolName,
 	ReputationChange,
 };
@@ -58,6 +59,7 @@ use std::{
 	io, iter,
 	ops::Deref,
 	pin::Pin,
+	sync::Arc,
 	task::{Context, Poll},
 	time::{Duration, Instant},
 };
@@ -129,11 +131,17 @@ pub struct ProtocolConfig {
 	pub inbound_queue: Option<async_channel::Sender<IncomingRequest>>,
 }
 
+impl RequestResponseConfigT for ProtocolConfig {
+	fn protocol_name(&self) -> &ProtocolName {
+		&self.name
+	}
+}
+
 /// A single request received by a peer on a request-response protocol.
 #[derive(Debug)]
 pub struct IncomingRequest {
 	/// Who sent the request.
-	pub peer: PeerId,
+	pub peer: sc_network_types::PeerId,
 
 	/// Request sent by the remote. Will always be smaller than
 	/// [`ProtocolConfig::max_request_size`].
@@ -288,7 +296,7 @@ pub struct RequestResponsesBehaviour {
 	send_feedback: HashMap<ProtocolRequestId, oneshot::Sender<()>>,
 
 	/// Primarily used to get a reputation of a node.
-	peer_store: Box<dyn PeerStoreProvider>,
+	peer_store: Arc<dyn PeerStoreProvider>,
 }
 
 /// Generated by the response builder and waiting to be processed.
@@ -305,7 +313,7 @@ impl RequestResponsesBehaviour {
 	/// the same protocol is passed twice.
 	pub fn new(
 		list: impl Iterator<Item = ProtocolConfig>,
-		peer_store: Box<dyn PeerStoreProvider>,
+		peer_store: Arc<dyn PeerStoreProvider>,
 	) -> Result<Self, RegisterError> {
 		let mut protocols = HashMap::new();
 		for protocol in list {
@@ -670,7 +678,7 @@ impl NetworkBehaviour for RequestResponsesBehaviour {
 							self.pending_responses_arrival_time
 								.insert((protocol.clone(), request_id).into(), Instant::now());
 
-							let reputation = self.peer_store.peer_reputation(&peer);
+							let reputation = self.peer_store.peer_reputation(&peer.into());
 
 							if reputation < BANNED_THRESHOLD {
 								log::debug!(
@@ -694,7 +702,7 @@ impl NetworkBehaviour for RequestResponsesBehaviour {
 								// because the latter allocates an extra slot for every cloned
 								// sender.
 								let _ = resp_builder.try_send(IncomingRequest {
-									peer,
+									peer: peer.into(),
 									payload: request,
 									pending_response: tx,
 								});
@@ -1093,7 +1101,7 @@ mod tests {
 			.multiplex(libp2p::yamux::Config::default())
 			.boxed();
 
-		let behaviour = RequestResponsesBehaviour::new(list, Box::new(MockPeerStore {})).unwrap();
+		let behaviour = RequestResponsesBehaviour::new(list, Arc::new(MockPeerStore {})).unwrap();
 
 		let runtime = tokio::runtime::Runtime::new().unwrap();
 		let mut swarm = SwarmBuilder::with_executor(