fix: fix resource leak (#136)

When a running service is dropped, its connections should be closed and its resources should be
cleaned up.

This changes the default from leaving services running to closing them. Library users who want
their services to remain running should call `waiting` on their running service, in a new task
where necessary.

Co-authored-by: Paul Ellenbogen <pe5@cs.princeton.edu>

examples/servers/src/generic_service.rs

@@ -13,6 +13,6 @@ async fn main() -> Result<(), Box<dyn Error>> {
 
     let io = (tokio::io::stdin(), tokio::io::stdout());
 
-    serve_server(generic_service, io).await?;
+    serve_server(generic_service, io).await?.waiting().await?;
     Ok(())
 }