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/network/test/src/block_import.rs

@@ -58,7 +58,7 @@ fn prepare_good_block() -> (TestClient, Hash, u64, PeerId, IncomingBlock<Block>)
 			body: Some(Vec::new()),
 			indexed_body: None,
 			justifications,
-			origin: Some(peer_id),
+			origin: Some(peer_id.into()),
 			allow_missing_state: false,
 			import_existing: false,
 			state: None,
@@ -81,7 +81,7 @@ fn import_single_good_block_works() {
 		&mut PassThroughVerifier::new(true),
 	)) {
 		Ok(BlockImportStatus::ImportedUnknown(ref num, ref aux, ref org))
-			if *num == number && *aux == expected_aux && *org == Some(peer_id) => {},
+			if *num == number && *aux == expected_aux && *org == Some(peer_id.into()) => {},
 		r @ _ => panic!("{:?}", r),
 	}
 }
@@ -110,7 +110,7 @@ fn import_single_good_block_without_header_fails() {
 		block,
 		&mut PassThroughVerifier::new(true),
 	)) {
-		Err(BlockImportError::IncompleteHeader(ref org)) if *org == Some(peer_id) => {},
+		Err(BlockImportError::IncompleteHeader(ref org)) if *org == Some(peer_id.into()) => {},
 		_ => panic!(),
 	}
 }