[tcp] Port-reuse, async-io, if-watch (#1887)

* Update tomls.

* Let transports decide when to translate.

* Improve tcp transport.

* Update stuff.

* Remove background task. Enhance documentation.

To avoid spawning a background task and thread within
`TcpConfig::new()`, with communication via unbounded channels,
a `TcpConfig` now keeps track of the listening addresses
for port reuse in an `Arc<RwLock>`. Furthermore, an `IfWatcher`
is only used by a `TcpListenStream` if it listens on any interface
and directly polls the `IfWatcher` both for initialisation and
new events.

Includes some documentation and test enhancements.

* Reintroduce feature flags for tokio vs async-io.

To avoid having an extra reactor thread running for tokio
users and to make sure all TCP I/O uses the mio-based
tokio reactor.

Thereby run tests with both backends.

* Add missing files.

* Fix docsrs attributes.

* Update transports/tcp/src/lib.rs

Co-authored-by: Max Inden <mail@max-inden.de>

* Restore chat-tokio example.

* Forward poll_write_vectored for tokio's AsyncWrite.

* Update changelogs.

Co-authored-by: David Craven <david@craven.ch>
Co-authored-by: Max Inden <mail@max-inden.de>

src/bandwidth.rs

@@ -74,6 +74,10 @@ where
             .dial(addr)
             .map(move |fut| BandwidthFuture { inner: fut, sinks })
     }
+
+    fn address_translation(&self, server: &Multiaddr, observed: &Multiaddr) -> Option<Multiaddr> {
+        self.inner.address_translation(server, observed)
+    }
 }
 
 /// Wraps around a `Stream` that produces connections. Wraps each connection around a bandwidth