Full support for multiple connections per peer in libp2p-swarm. (#1519)

* [libp2p-swarm] Make the multiple connections per peer first-class.

This commit makes the notion of multiple connections per peer
first-class in the API of libp2p-swarm, introducing the new
callbacks `inject_connection_established` and
`inject_connection_closed`. The `endpoint` parameter from
`inject_connected` and `inject_disconnected` is removed,
since the first connection to open may not be the last
connection to close, i.e. it cannot be guaranteed,
as was previously the case, that the endpoints passed
to these callbacks match up.

* Have identify track all addresses.

So that identify requests can be answered with the correct
observed address of the connection on which the request
arrives.

* Cleanup

* Cleanup

* Improve the `Peer` state API.

* Remove connection ID from `SwarmEvent::Dialing`.

* Mark `DialPeerCondition` non-exhaustive.

* Re-encapsulate `NetworkConfig`.

To retain the possibility of not re-exposing all
network configuration choices, thereby providing
a more convenient API on the \`SwarmBuilder\`.

* Rework Swarm::dial API.

* Update CHANGELOG.

* Doc formatting tweaks.

swarm/src/protocols_handler/node_handler.rs

@@ -133,10 +133,10 @@ enum Shutdown {
 /// Error generated by the `NodeHandlerWrapper`.
 #[derive(Debug)]
 pub enum NodeHandlerWrapperError<TErr> {
-    /// Error generated by the handler.
+    /// The connection handler encountered an error.
     Handler(TErr),
-    /// The connection has been deemed useless and has been closed.
-    UselessTimeout,
+    /// The connection keep-alive timeout expired.
+    KeepAliveTimeout,
 }
 
 impl<TErr> From<TErr> for NodeHandlerWrapperError<TErr> {
@@ -152,8 +152,8 @@ where
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             NodeHandlerWrapperError::Handler(err) => write!(f, "{}", err),
-            NodeHandlerWrapperError::UselessTimeout =>
-                write!(f, "Node has been closed due to inactivity"),
+            NodeHandlerWrapperError::KeepAliveTimeout =>
+                write!(f, "Connection closed due to expired keep-alive timeout."),
         }
     }
 }
@@ -165,7 +165,7 @@ where
     fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match self {
             NodeHandlerWrapperError::Handler(err) => Some(err),
-            NodeHandlerWrapperError::UselessTimeout => None,
+            NodeHandlerWrapperError::KeepAliveTimeout => None,
         }
     }
 }
@@ -314,9 +314,9 @@ where
         if self.negotiating_in.is_empty() && self.negotiating_out.is_empty() {
             match self.shutdown {
                 Shutdown::None => {},
-                Shutdown::Asap => return Poll::Ready(Err(NodeHandlerWrapperError::UselessTimeout)),
+                Shutdown::Asap => return Poll::Ready(Err(NodeHandlerWrapperError::KeepAliveTimeout)),
                 Shutdown::Later(ref mut delay, _) => match Future::poll(Pin::new(delay), cx) {
-                    Poll::Ready(_) => return Poll::Ready(Err(NodeHandlerWrapperError::UselessTimeout)),
+                    Poll::Ready(_) => return Poll::Ready(Err(NodeHandlerWrapperError::KeepAliveTimeout)),
                     Poll::Pending => {}
                 }
             }