fix: smooth progress bar for backup transfer

Before this change progress bar only started
when database is already transferred.
Database is usually the largest file
in the whole transfer, so the transfer appears
to be stuck for the sender.

With this change progress bar
starts for backup export
as soon as connection is received
and counts bytes transferred over the connection
using AsyncWrite wrapper.

Similarly for backup import,
AsyncRead wrapper counts the bytes
received and emits progress events.

src/blob.rs

@@ -666,10 +666,6 @@ impl<'a> BlobDirContents<'a> {
     pub(crate) fn iter(&self) -> BlobDirIter<'_> {
         BlobDirIter::new(self.context, self.inner.iter())
     }
-
-    pub(crate) fn len(&self) -> usize {
-        self.inner.len()
-    }
 }
 
 /// A iterator over all the [`BlobObject`]s in the blobdir.