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/sync/src/state_request_handler.rs

@@ -24,15 +24,16 @@ use crate::{
 
 use codec::{Decode, Encode};
 use futures::{channel::oneshot, stream::StreamExt};
-use libp2p::PeerId;
 use log::{debug, trace};
 use prost::Message;
+use sc_network_types::PeerId;
 use schnellru::{ByLength, LruMap};
 
 use sc_client_api::{BlockBackend, ProofProvider};
 use sc_network::{
 	config::ProtocolId,
-	request_responses::{IncomingRequest, OutgoingResponse, ProtocolConfig},
+	request_responses::{IncomingRequest, OutgoingResponse},
+	NetworkBackend,
 };
 use sp_runtime::traits::Block as BlockT;
 
@@ -52,21 +53,26 @@ mod rep {
 	pub const SAME_REQUEST: Rep = Rep::new(i32::MIN, "Same state request multiple times");
 }
 
-/// Generates a [`ProtocolConfig`] for the state request protocol, refusing incoming requests.
-pub fn generate_protocol_config<Hash: AsRef<[u8]>>(
+/// Generates a `RequestResponseProtocolConfig` for the state request protocol, refusing incoming
+/// requests.
+pub fn generate_protocol_config<
+	Hash: AsRef<[u8]>,
+	B: BlockT,
+	N: NetworkBackend<B, <B as BlockT>::Hash>,
+>(
 	protocol_id: &ProtocolId,
 	genesis_hash: Hash,
 	fork_id: Option<&str>,
-) -> ProtocolConfig {
-	ProtocolConfig {
-		name: generate_protocol_name(genesis_hash, fork_id).into(),
-		fallback_names: std::iter::once(generate_legacy_protocol_name(protocol_id).into())
-			.collect(),
-		max_request_size: 1024 * 1024,
-		max_response_size: 16 * 1024 * 1024,
-		request_timeout: Duration::from_secs(40),
-		inbound_queue: None,
-	}
+	inbound_queue: async_channel::Sender<IncomingRequest>,
+) -> N::RequestResponseProtocolConfig {
+	N::request_response_config(
+		generate_protocol_name(genesis_hash, fork_id).into(),
+		std::iter::once(generate_legacy_protocol_name(protocol_id).into()).collect(),
+		1024 * 1024,
+		16 * 1024 * 1024,
+		Duration::from_secs(40),
+		Some(inbound_queue),
+	)
 }
 
 /// Generate the state protocol name from the genesis hash and fork id.
@@ -125,18 +131,18 @@ where
 	Client: BlockBackend<B> + ProofProvider<B> + Send + Sync + 'static,
 {
 	/// Create a new [`StateRequestHandler`].
-	pub fn new(
+	pub fn new<N: NetworkBackend<B, <B as BlockT>::Hash>>(
 		protocol_id: &ProtocolId,
 		fork_id: Option<&str>,
 		client: Arc<Client>,
 		num_peer_hint: usize,
-	) -> (Self, ProtocolConfig) {
+	) -> (Self, N::RequestResponseProtocolConfig) {
 		// Reserve enough request slots for one request per peer when we are at the maximum
 		// number of peers.
 		let capacity = std::cmp::max(num_peer_hint, 1);
 		let (tx, request_receiver) = async_channel::bounded(capacity);
 
-		let mut protocol_config = generate_protocol_config(
+		let protocol_config = generate_protocol_config::<_, B, N>(
 			protocol_id,
 			client
 				.block_hash(0u32.into())
@@ -144,8 +150,8 @@ where
 				.flatten()
 				.expect("Genesis block exists; qed"),
 			fork_id,
+			tx,
 		);
-		protocol_config.inbound_queue = Some(tx);
 
 		let capacity = ByLength::new(num_peer_hint.max(1) as u32 * 2);
 		let seen_requests = LruMap::new(capacity);