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>

polkadot/node/test/service/src/lib.rs

@@ -79,24 +79,46 @@ pub fn new_full<OverseerGenerator: OverseerGen>(
 ) -> Result<NewFull, Error> {
 	let workers_path = Some(workers_path.unwrap_or_else(get_relative_workers_path_for_test));
 
-	polkadot_service::new_full(
-		config,
-		polkadot_service::NewFullParams {
-			is_parachain_node,
-			enable_beefy: true,
-			force_authoring_backoff: false,
-			jaeger_agent: None,
-			telemetry_worker_handle: None,
-			node_version: None,
-			secure_validator_mode: false,
-			workers_path,
-			workers_names: None,
-			overseer_gen,
-			overseer_message_channel_capacity_override: None,
-			malus_finality_delay: None,
-			hwbench: None,
-		},
-	)
+	match config.network.network_backend {
+		sc_network::config::NetworkBackendType::Libp2p =>
+			polkadot_service::new_full::<_, sc_network::NetworkWorker<_, _>>(
+				config,
+				polkadot_service::NewFullParams {
+					is_parachain_node,
+					enable_beefy: true,
+					force_authoring_backoff: false,
+					jaeger_agent: None,
+					telemetry_worker_handle: None,
+					node_version: None,
+					secure_validator_mode: false,
+					workers_path,
+					workers_names: None,
+					overseer_gen,
+					overseer_message_channel_capacity_override: None,
+					malus_finality_delay: None,
+					hwbench: None,
+				},
+			),
+		sc_network::config::NetworkBackendType::Litep2p =>
+			polkadot_service::new_full::<_, sc_network::Litep2pNetworkBackend>(
+				config,
+				polkadot_service::NewFullParams {
+					is_parachain_node,
+					enable_beefy: true,
+					force_authoring_backoff: false,
+					jaeger_agent: None,
+					telemetry_worker_handle: None,
+					node_version: None,
+					secure_validator_mode: false,
+					workers_path,
+					workers_names: None,
+					overseer_gen,
+					overseer_message_channel_capacity_override: None,
+					malus_finality_delay: None,
+					hwbench: None,
+				},
+			),
+	}
 }
 
 fn get_relative_workers_path_for_test() -> PathBuf {