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/telemetry/src/endpoints.rs

@@ -16,7 +16,7 @@
 // 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 libp2p::Multiaddr;
+use sc_network::{multiaddr, Multiaddr};
 use serde::{Deserialize, Deserializer, Serialize};
 
 /// List of telemetry servers we want to talk to. Contains the URL of the server, and the
@@ -41,8 +41,8 @@ where
 
 impl TelemetryEndpoints {
 	/// Create a `TelemetryEndpoints` based on a list of `(String, u8)`.
-	pub fn new(endpoints: Vec<(String, u8)>) -> Result<Self, libp2p::multiaddr::Error> {
-		let endpoints: Result<Vec<(Multiaddr, u8)>, libp2p::multiaddr::Error> =
+	pub fn new(endpoints: Vec<(String, u8)>) -> Result<Self, multiaddr::Error> {
+		let endpoints: Result<Vec<(Multiaddr, u8)>, multiaddr::Error> =
 			endpoints.iter().map(|e| Ok((url_to_multiaddr(&e.0)?, e.1))).collect();
 		endpoints.map(Self)
 	}
@@ -56,7 +56,7 @@ impl TelemetryEndpoints {
 }
 
 /// Parses a WebSocket URL into a libp2p `Multiaddr`.
-fn url_to_multiaddr(url: &str) -> Result<Multiaddr, libp2p::multiaddr::Error> {
+fn url_to_multiaddr(url: &str) -> Result<Multiaddr, multiaddr::Error> {
 	// First, assume that we have a `Multiaddr`.
 	let parse_error = match url.parse() {
 		Ok(ma) => return Ok(ma),
@@ -64,7 +64,7 @@ fn url_to_multiaddr(url: &str) -> Result<Multiaddr, libp2p::multiaddr::Error> {
 	};
 
 	// If not, try the `ws://path/url` format.
-	if let Ok(ma) = libp2p::multiaddr::from_url(url) {
+	if let Ok(ma) = multiaddr::from_url(url) {
 		return Ok(ma)
 	}
 
@@ -75,8 +75,7 @@ fn url_to_multiaddr(url: &str) -> Result<Multiaddr, libp2p::multiaddr::Error> {
 
 #[cfg(test)]
 mod tests {
-	use super::{url_to_multiaddr, TelemetryEndpoints};
-	use libp2p::Multiaddr;
+	use super::{url_to_multiaddr, Multiaddr, TelemetryEndpoints};
 
 	#[test]
 	fn valid_endpoints() {