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/warp_request_handler.rs

@@ -26,9 +26,8 @@ use crate::{
 };
 use sc_network::{
 	config::ProtocolId,
-	request_responses::{
-		IncomingRequest, OutgoingResponse, ProtocolConfig as RequestResponseConfig,
-	},
+	request_responses::{IncomingRequest, OutgoingResponse},
+	NetworkBackend,
 };
 use sp_runtime::traits::Block as BlockT;
 
@@ -39,22 +38,26 @@ const MAX_RESPONSE_SIZE: u64 = 16 * 1024 * 1024;
 /// Incoming warp requests bounded queue size.
 const MAX_WARP_REQUEST_QUEUE: usize = 20;
 
-/// Generates a [`RequestResponseConfig`] for the grandpa warp sync request protocol, refusing
+/// Generates a `RequestResponseProtocolConfig` for the grandpa warp sync request protocol, refusing
 /// incoming requests.
-pub fn generate_request_response_config<Hash: AsRef<[u8]>>(
+pub fn generate_request_response_config<
+	Hash: AsRef<[u8]>,
+	B: BlockT,
+	N: NetworkBackend<B, <B as BlockT>::Hash>,
+>(
 	protocol_id: ProtocolId,
 	genesis_hash: Hash,
 	fork_id: Option<&str>,
-) -> RequestResponseConfig {
-	RequestResponseConfig {
-		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: 32,
-		max_response_size: MAX_RESPONSE_SIZE,
-		request_timeout: Duration::from_secs(10),
-		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(),
+		32,
+		MAX_RESPONSE_SIZE,
+		Duration::from_secs(10),
+		Some(inbound_queue),
+	)
 }
 
 /// Generate the grandpa warp sync protocol name from the genesis hash and fork id.
@@ -80,17 +83,20 @@ pub struct RequestHandler<TBlock: BlockT> {
 
 impl<TBlock: BlockT> RequestHandler<TBlock> {
 	/// Create a new [`RequestHandler`].
-	pub fn new<Hash: AsRef<[u8]>>(
+	pub fn new<Hash: AsRef<[u8]>, N: NetworkBackend<TBlock, <TBlock as BlockT>::Hash>>(
 		protocol_id: ProtocolId,
 		genesis_hash: Hash,
 		fork_id: Option<&str>,
 		backend: Arc<dyn WarpSyncProvider<TBlock>>,
-	) -> (Self, RequestResponseConfig) {
+	) -> (Self, N::RequestResponseProtocolConfig) {
 		let (tx, request_receiver) = async_channel::bounded(MAX_WARP_REQUEST_QUEUE);
 
-		let mut request_response_config =
-			generate_request_response_config(protocol_id, genesis_hash, fork_id);
-		request_response_config.inbound_queue = Some(tx);
+		let request_response_config = generate_request_response_config::<_, TBlock, N>(
+			protocol_id,
+			genesis_hash,
+			fork_id,
+			tx,
+		);
 
 		(Self { backend, request_receiver }, request_response_config)
 	}