feat: Pause IO for BackupProvider (#4182)

This makes the BackupProvider automatically invoke pause-io while it
is needed.

It needed to make the guard independent from the Context lifetime to
make this work.  Which is a bit sad.

src/imex.rs

@@ -91,7 +91,7 @@ pub async fn imex(
     let cancel = context.alloc_ongoing().await?;
 
     let res = {
-        let mut guard = context.scheduler.pause(context).await;
+        let mut guard = context.scheduler.pause(context.clone()).await;
         let res = imex_inner(context, what, path, passphrase)
             .race(async {
                 cancel.recv().await.ok();