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>

cumulus/client/service/src/lib.rs

@@ -40,7 +40,7 @@ use sc_consensus::{
 	import_queue::{ImportQueue, ImportQueueService},
 	BlockImport,
 };
-use sc_network::{config::SyncMode, NetworkService};
+use sc_network::{config::SyncMode, service::traits::NetworkService, NetworkBackend};
 use sc_network_sync::SyncingService;
 use sc_network_transactions::TransactionsHandlerController;
 use sc_service::{Configuration, NetworkStarter, SpawnTaskHandle, TaskManager, WarpSyncParams};
@@ -406,13 +406,15 @@ pub struct BuildNetworkParams<
 		+ HeaderBackend<Block>
 		+ BlockIdTo<Block>
 		+ 'static,
+	Network: NetworkBackend<Block, <Block as BlockT>::Hash>,
 	RCInterface,
 	IQ,
 > where
 	Client::Api: sp_transaction_pool::runtime_api::TaggedTransactionQueue<Block>,
 {
 	pub parachain_config: &'a Configuration,
-	pub net_config: sc_network::config::FullNetworkConfiguration,
+	pub net_config:
+		sc_network::config::FullNetworkConfiguration<Block, <Block as BlockT>::Hash, Network>,
 	pub client: Arc<Client>,
 	pub transaction_pool: Arc<sc_transaction_pool::FullPool<Block, Client>>,
 	pub para_id: ParaId,
@@ -423,7 +425,7 @@ pub struct BuildNetworkParams<
 }
 
 /// Build the network service, the network status sinks and an RPC sender.
-pub async fn build_network<'a, Block, Client, RCInterface, IQ>(
+pub async fn build_network<'a, Block, Client, RCInterface, IQ, Network>(
 	BuildNetworkParams {
 		parachain_config,
 		net_config,
@@ -434,9 +436,9 @@ pub async fn build_network<'a, Block, Client, RCInterface, IQ>(
 		relay_chain_interface,
 		import_queue,
 		sybil_resistance_level,
-	}: BuildNetworkParams<'a, Block, Client, RCInterface, IQ>,
+	}: BuildNetworkParams<'a, Block, Client, Network, RCInterface, IQ>,
 ) -> sc_service::error::Result<(
-	Arc<NetworkService<Block, Block::Hash>>,
+	Arc<dyn NetworkService>,
 	TracingUnboundedSender<sc_rpc::system::Request<Block>>,
 	TransactionsHandlerController<Block::Hash>,
 	NetworkStarter,
@@ -461,6 +463,7 @@ where
 	for<'b> &'b Client: BlockImport<Block>,
 	RCInterface: RelayChainInterface + Clone + 'static,
 	IQ: ImportQueue<Block> + 'static,
+	Network: NetworkBackend<Block, <Block as BlockT>::Hash>,
 {
 	let warp_sync_params = match parachain_config.network.sync_mode {
 		SyncMode::Warp => {
@@ -485,6 +488,9 @@ where
 			Box::new(block_announce_validator) as Box<_>
 		},
 	};
+	let metrics = Network::register_notification_metrics(
+		parachain_config.prometheus_config.as_ref().map(|cfg| &cfg.registry),
+	);
 
 	sc_service::build_network(sc_service::BuildNetworkParams {
 		config: parachain_config,
@@ -496,6 +502,7 @@ where
 		block_announce_validator_builder: Some(Box::new(move |_| block_announce_validator)),
 		warp_sync_params,
 		block_relay: None,
+		metrics,
 	})
 }