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/mixnet/src/run.rs

@@ -44,8 +44,8 @@ use mixnet::{
 };
 use sc_client_api::{BlockchainEvents, HeaderBackend};
 use sc_network::{
-	service::traits::{NotificationEvent, ValidationResult},
-	NetworkNotification, NetworkPeers, NetworkStateInfo, NotificationService, ProtocolName,
+	service::traits::{NetworkService, NotificationEvent, ValidationResult},
+	NetworkPeers, NetworkStateInfo, NotificationService, ProtocolName,
 };
 use sc_transaction_pool_api::{
 	LocalTransactionPool, OffchainTransactionPoolFactory, TransactionPool,
@@ -146,12 +146,12 @@ fn time_until(instant: Instant) -> Duration {
 
 /// Run the mixnet service. If `keystore` is `None`, the service will not attempt to register the
 /// local node as a mixnode, even if `config.register` is `true`.
-pub async fn run<B, C, S, N, P>(
+pub async fn run<B, C, S, P>(
 	config: Config,
 	mut api_backend: ApiBackend,
 	client: Arc<C>,
 	sync: Arc<S>,
-	network: Arc<N>,
+	network: Arc<dyn NetworkService>,
 	protocol_name: ProtocolName,
 	transaction_pool: Arc<P>,
 	keystore: Option<KeystorePtr>,
@@ -161,7 +161,6 @@ pub async fn run<B, C, S, N, P>(
 	C: BlockchainEvents<B> + ProvideRuntimeApi<B> + HeaderBackend<B>,
 	C::Api: MixnetApi<B>,
 	S: SyncOracle,
-	N: NetworkStateInfo + NetworkNotification + NetworkPeers,
 	P: TransactionPool<Block = B> + LocalTransactionPool<Block = B> + 'static,
 {
 	let local_peer_id = network.local_peer_id();