protocols/relay: Implement circuit relay specification (#1838)

This commit implements the [libp2p circuit
relay](https://github.com/libp2p/specs/tree/master/relay) specification. It is
based on previous work from https://github.com/libp2p/rust-libp2p/pull/1134.

Instead of altering the `Transport` trait, the approach taken in this commit
is to wrap an existing implementation of `Transport` allowing one to:

- Intercept `dial` requests with a relayed address.

- Inject incoming relayed connections with the local node being the destination.

- Intercept `listen_on` requests pointing to a relay, ensuring to keep a
  constant connection to the relay, waiting for incoming requests with the local
  node being the destination.

More concretely one would wrap an existing `Transport` implementation as seen
below, allowing the `Relay` behaviour and the `RelayTransport` to communicate
via channels.

### Example

```rust
let (relay_transport, relay_behaviour) = new_transport_and_behaviour(
    RelayConfig::default(),
    MemoryTransport::default(),
);

let transport = relay_transport
    .upgrade(upgrade::Version::V1)
    .authenticate(plaintext)
    .multiplex(YamuxConfig::default())
    .boxed();

let mut swarm = Swarm::new(transport, relay_behaviour, local_peer_id);

let relay_addr = Multiaddr::from_str("/memory/1234").unwrap()
    .with(Protocol::P2p(PeerId::random().into()))
    .with(Protocol::P2pCircuit);
let dst_addr = relay_addr.clone().with(Protocol::Memory(5678));

// Listen for incoming connections via relay node (1234).
Swarm::listen_on(&mut swarm, relay_addr).unwrap();

// Dial node (5678) via relay node (1234).
Swarm::dial_addr(&mut swarm, dst_addr).unwrap();
```

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>
Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>
Co-authored-by: David Craven <david@craven.ch>

protocols/relay/examples/relay.rs

@@ -0,0 +1,84 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+use futures::executor::block_on;
+use futures::stream::StreamExt;
+use libp2p::core::upgrade;
+use libp2p::plaintext;
+use libp2p::relay::RelayConfig;
+use libp2p::tcp::TcpConfig;
+use libp2p::Transport;
+use libp2p::{identity, PeerId, Swarm};
+use std::error::Error;
+use std::task::{Context, Poll};
+use std::time::Duration;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    env_logger::init();
+
+    // Create a random PeerId
+    let local_key = identity::Keypair::generate_ed25519();
+    let local_peer_id = PeerId::from(local_key.public());
+    println!("Local peer id: {:?}", local_peer_id);
+
+    let tcp_transport = TcpConfig::new();
+
+    let relay_config = RelayConfig {
+        connection_idle_timeout: Duration::from_secs(10 * 60),
+        ..Default::default()
+    };
+    let (relay_wrapped_transport, relay_behaviour) =
+        libp2p_relay::new_transport_and_behaviour(relay_config, tcp_transport);
+
+    let plaintext = plaintext::PlainText2Config {
+        local_public_key: local_key.public(),
+    };
+
+    let transport = relay_wrapped_transport
+        .upgrade(upgrade::Version::V1)
+        .authenticate(plaintext)
+        .multiplex(libp2p_yamux::YamuxConfig::default())
+        .boxed();
+
+    let mut swarm = Swarm::new(transport, relay_behaviour, local_peer_id);
+
+    // Listen on all interfaces and whatever port the OS assigns
+    Swarm::listen_on(&mut swarm, "/ip6/::/tcp/0".parse()?)?;
+
+    let mut listening = false;
+    block_on(futures::future::poll_fn(move |cx: &mut Context<'_>| {
+        loop {
+            match swarm.poll_next_unpin(cx) {
+                Poll::Ready(Some(event)) => println!("{:?}", event),
+                Poll::Ready(None) => return Poll::Ready(Ok(())),
+                Poll::Pending => {
+                    if !listening {
+                        for addr in Swarm::listeners(&swarm) {
+                            println!("Listening on {:?}", addr);
+                            listening = true;
+                        }
+                    }
+                    break;
+                }
+            }
+        }
+        Poll::Pending
+    }))
+}