[libp2p-dns] Implement /dnsaddr resolution. (#1931)

* Implement `/dnsaddr` support on `libp2p-dns`.

To that end, since resolving `/dnsaddr` addresses needs
"fully qualified" multiaddresses when dialing, i.e. those
that end with the `/p2p/...` protocol, we make sure that
dialing always uses such fully qualified addresses by
appending the `/p2p` protocol as necessary. As a side-effect,
this adds support for dialing peers via "fully qualified"
addresses, as an alternative to using a `PeerId` together
with a `Multiaddr` with or without the `/p2p` protocol.

* Adapt libp2p-relay.

* Update versions, changelogs and small cleanups.

misc/multiaddr/src/lib.rs

@@ -174,6 +174,16 @@ impl Multiaddr {
 
         if replaced { Some(address) } else { None }
     }
+
+    /// Checks whether the given `Multiaddr` is a suffix of this `Multiaddr`.
+    pub fn ends_with(&self, other: &Multiaddr) -> bool {
+        let n = self.bytes.len();
+        let m = other.bytes.len();
+        if n < m {
+            return false
+        }
+        self.bytes[(n - m) ..] == other.bytes[..]
+    }
 }
 
 impl fmt::Debug for Multiaddr {