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/service/test/src/lib.rs

@@ -73,9 +73,7 @@ pub trait TestNetNode: Clone + Future<Output = Result<(), Error>> + Send + 'stat
 
 	fn client(&self) -> Arc<Client<Self::Backend, Self::Executor, Self::Block, Self::RuntimeApi>>;
 	fn transaction_pool(&self) -> Arc<Self::TransactionPool>;
-	fn network(
-		&self,
-	) -> Arc<sc_network::NetworkService<Self::Block, <Self::Block as BlockT>::Hash>>;
+	fn network(&self) -> Arc<dyn sc_network::service::traits::NetworkService>;
 	fn sync(&self) -> &Arc<SyncingService<Self::Block>>;
 	fn spawn_handle(&self) -> SpawnTaskHandle;
 }
@@ -84,7 +82,7 @@ pub struct TestNetComponents<TBl: BlockT, TBackend, TExec, TRtApi, TExPool> {
 	task_manager: Arc<Mutex<TaskManager>>,
 	client: Arc<Client<TBackend, TExec, TBl, TRtApi>>,
 	transaction_pool: Arc<TExPool>,
-	network: Arc<sc_network::NetworkService<TBl, <TBl as BlockT>::Hash>>,
+	network: Arc<dyn sc_network::service::traits::NetworkService>,
 	sync: Arc<SyncingService<TBl>>,
 }
 
@@ -94,7 +92,7 @@ impl<TBl: BlockT, TBackend, TExec, TRtApi, TExPool>
 	pub fn new(
 		task_manager: TaskManager,
 		client: Arc<Client<TBackend, TExec, TBl, TRtApi>>,
-		network: Arc<sc_network::NetworkService<TBl, <TBl as BlockT>::Hash>>,
+		network: Arc<dyn sc_network::service::traits::NetworkService>,
 		sync: Arc<SyncingService<TBl>>,
 		transaction_pool: Arc<TExPool>,
 	) -> Self {
@@ -153,9 +151,7 @@ where
 	fn transaction_pool(&self) -> Arc<Self::TransactionPool> {
 		self.transaction_pool.clone()
 	}
-	fn network(
-		&self,
-	) -> Arc<sc_network::NetworkService<Self::Block, <Self::Block as BlockT>::Hash>> {
+	fn network(&self) -> Arc<dyn sc_network::service::traits::NetworkService> {
 		self.network.clone()
 	}
 	fn sync(&self) -> &Arc<SyncingService<Self::Block>> {