Add listener closing when the target set has changed.

src/listener.rs

@@ -1,9 +1,11 @@
 use crate::shutdown::ShutdownReceiver;
 use std::sync::Arc;
+use tokio::sync::broadcast;
 use tokio::sync::RwLock;
 
 pub(crate) struct Listener {
     pub(crate) shutdown: ShutdownReceiver,
     pub(crate) source: String,
     pub(crate) targets: Arc<RwLock<Vec<String>>>,
+    pub(crate) targets_changed: broadcast::Receiver<()>,
 }