Shutdown like a Sir (#1545)

## Issue Addressed
#1494 

## Proposed Changes
- Give the TaskExecutor the sender side of a channel that a task can clone to request shutting down
- The receiver side of this channel is in environment and now we block until ctrl+c or an internal shutdown signal is received
- The swarm now informs when it has reached 0 listeners
- The network receives this message and requests the shutdown

beacon_node/network/src/service.rs

@@ -169,6 +169,7 @@ fn spawn_service<T: BeaconChainTypes>(
     mut service: NetworkService<T>,
 ) -> error::Result<()> {
     let mut exit_rx = executor.exit();
+    let mut shutdown_sender = executor.shutdown_sender();
 
     // spawn on the current executor
     executor.spawn_without_exit(async move {
@@ -376,6 +377,12 @@ fn spawn_service<T: BeaconChainTypes>(
                         Libp2pEvent::NewListenAddr(multiaddr) => {
                             service.network_globals.listen_multiaddrs.write().push(multiaddr);
                         }
+                        Libp2pEvent::ZeroListeners => {
+                            let _ = shutdown_sender.send("All listeners are closed. Unable to listen").await.map_err(|e| {
+                                warn!(service.log, "failed to send a shutdown signal"; "error" => e.to_string()
+                                )
+                            });
+                        }
                     }
                 }
             }