feat: Make the IoPausedGuard a simple sender (#4184)

This replaces the mechanism by which the IoPauseGuard makes sure the
IO scheduler is resumed: it really is a drop guard now by sending a
single message on drop.

This makes it not have to hold on to anything like the context so
makes it a lot easier to use.

The trade-off is that a long-running task is spawned when the guard is
created, this task needs to receive the message from the drop guard in
order for the scheduler to resume.

src/imex.rs

@@ -91,15 +91,13 @@ pub async fn imex(
     let cancel = context.alloc_ongoing().await?;
 
     let res = {
-        let mut guard = context.scheduler.pause(context.clone()).await;
-        let res = imex_inner(context, what, path, passphrase)
+        let _guard = context.scheduler.pause(context.clone()).await;
+        imex_inner(context, what, path, passphrase)
             .race(async {
                 cancel.recv().await.ok();
                 Err(format_err!("canceled"))
             })
-            .await;
-        guard.resume().await;
-        res
+            .await
     };
     context.free_ongoing().await;