ref: make Context::alloc_ongoing return a guard

This guard can be awaited and will resolve when Context::stop_ongoing
is called, i.e. the ongoing process is cancelled.  The guard will also
free the ongoing process when dropped, making it RAII and easier to
not accidentally free the ongoing process.

src/imex.rs

@@ -94,7 +94,7 @@ pub async fn imex(
         let _guard = context.scheduler.pause(context.clone()).await;
         imex_inner(context, what, path, passphrase)
             .race(async {
-                cancel.recv().await.ok();
+                cancel.await;
                 Err(format_err!("canceled"))
             })
             .await