Use loop to poll FuturesUnordered after push. (#198)

Fixes #190.

core/src/swarm.rs

@@ -268,6 +268,7 @@ where
             Ok(Async::NotReady) => {}
         };
 
+        loop {
             match self.listeners.poll() {
                 Ok(Async::Ready(Some((Some(upgrade), remaining)))) => {
                     trace!("Swarm received new connection on listener socket");
@@ -276,8 +277,10 @@ where
                 }
                 Err((err, _)) => {
                     warn!("Error in listener: {:?}", err);
+                    break
+                }
+                _ => break
             }
-            _ => {}
         }
 
         match self.listeners_upgrade.poll() {