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/src/schema/bitswap.v1.2.0.proto

@@ -0,0 +1,43 @@
+syntax = "proto3";
+
+package bitswap.message;
+
+message Message {
+	message Wantlist {
+		enum WantType {
+			Block = 0;
+			Have = 1;
+		}
+
+		message Entry {
+			bytes block = 1; 	// the block cid (cidV0 in bitswap 1.0.0, cidV1 in bitswap 1.1.0)
+			int32 priority = 2;	// the priority (normalized). default to 1
+			bool cancel = 3;		// whether this revokes an entry
+			WantType wantType = 4; // Note: defaults to enum 0, ie Block
+			bool sendDontHave = 5; // Note: defaults to false
+		}
+
+		repeated Entry entries = 1; // a list of wantlist entries
+		bool full = 2;							// whether this is the full wantlist. default to false
+	}
+
+	message Block {
+		bytes prefix = 1;		// CID prefix (cid version, multicodec and multihash prefix (type + length)
+		bytes data = 2;
+	}
+
+	enum BlockPresenceType {
+		Have = 0;
+		DontHave = 1;
+	}
+	message BlockPresence {
+		bytes cid = 1;
+		BlockPresenceType type = 2;
+	}
+
+	Wantlist wantlist = 1;
+	repeated bytes blocks = 2;		// used to send Blocks in bitswap 1.0.0
+	repeated Block payload = 3;		// used to send Blocks in bitswap 1.1.0
+	repeated BlockPresence blockPresences = 4;
+	int32 pendingBytes = 5;
+}