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/cli/src/arg_enums.rs

@@ -296,3 +296,23 @@ impl Into<sc_network::config::SyncMode> for SyncMode {
 		}
 	}
 }
+
+/// Network backend type.
+#[derive(Debug, Clone, Copy, ValueEnum, PartialEq)]
+#[value(rename_all = "lower")]
+pub enum NetworkBackendType {
+	/// Use libp2p for P2P networking.
+	Libp2p,
+
+	/// Use litep2p for P2P networking.
+	Litep2p,
+}
+
+impl Into<sc_network::config::NetworkBackendType> for NetworkBackendType {
+	fn into(self) -> sc_network::config::NetworkBackendType {
+		match self {
+			Self::Libp2p => sc_network::config::NetworkBackendType::Libp2p,
+			Self::Litep2p => sc_network::config::NetworkBackendType::Litep2p,
+		}
+	}
+}

substrate/client/cli/src/commands/build_spec_cmd.rs

@@ -67,7 +67,7 @@ impl BuildSpecCmd {
 			let peer_id = keys.public().to_peer_id();
 			let addr = MultiaddrWithPeerId {
 				multiaddr: build_multiaddr![Ip4([127, 0, 0, 1]), Tcp(30333u16)],
-				peer_id,
+				peer_id: peer_id.into(),
 			};
 			spec.add_boot_node(addr)
 		}

substrate/client/cli/src/params/network_params.rs

@@ -16,7 +16,10 @@
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
-use crate::{arg_enums::SyncMode, params::node_key_params::NodeKeyParams};
+use crate::{
+	arg_enums::{NetworkBackendType, SyncMode},
+	params::node_key_params::NodeKeyParams,
+};
 use clap::Args;
 use sc_network::{
 	config::{
@@ -166,6 +169,20 @@ pub struct NetworkParams {
 	/// and observe block requests timing out.
 	#[arg(long, value_name = "COUNT", default_value_t = 64)]
 	pub max_blocks_per_request: u32,
+
+	/// Network backend used for P2P networking.
+	///
+	/// litep2p network backend is considered experimental and isn't as stable as the libp2p
+	/// network backend.
+	#[arg(
+		long,
+		value_enum,
+		value_name = "NETWORK_BACKEND",
+		default_value_t = NetworkBackendType::Libp2p,
+		ignore_case = true,
+		verbatim_doc_comment
+	)]
+	pub network_backend: NetworkBackendType,
 }
 
 impl NetworkParams {
@@ -261,6 +278,7 @@ impl NetworkParams {
 			yamux_window_size: None,
 			ipfs_server: self.ipfs_server,
 			sync_mode: self.sync.into(),
+			network_backend: self.network_backend.into(),
 		}
 	}
 }