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/tests.rs

@@ -40,7 +40,13 @@ mod tests {
         let runtime = Runtime::new().unwrap();
 
         let (signal, exit) = exit_future::signal();
-        let executor = environment::TaskExecutor::new(runtime.handle().clone(), exit, log.clone());
+        let (shutdown_tx, _) = futures::channel::mpsc::channel(1);
+        let executor = environment::TaskExecutor::new(
+            runtime.handle().clone(),
+            exit,
+            log.clone(),
+            shutdown_tx,
+        );
 
         let mut config = NetworkConfig::default();
         config.libp2p_port = 21212;