fix(imex): flush each tar entry immediately after appending

When streaming over the network we don't want
to buffer tar entries, but send them immediately.

This is an attempt to fix timeout errors on the receiver side
when transferring large files that look like

Failed to import backup from QUIC stream: Failed to unpack file: failed to unpack `/home/user/.config/DeltaChat/accounts/<uuid>/dc.db-blobs/blobs_backup/$BLOBDIR/<filename>.mp4`: failed to unpack `blobs_backup/$BLOBDIR/<filename>.mp4` into `/home/user/.config/DeltaChat/accounts/<uuid>/dc.db-blobs/blobs_backup/$BLOBDIR/<filename>.mp4`: connection lost: timed out"

src/imex.rs

@@ -10,7 +10,7 @@ use futures_lite::FutureExt;
 use pin_project::pin_project;
 
 use tokio::fs::{self, File};
-use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
+use tokio::io::{AsyncRead, AsyncWrite, AsyncWriteExt, ReadBuf};
 use tokio_tar::Archive;
 
 use crate::blob::BlobDirContents;
@@ -575,6 +575,14 @@ where
         let mut file = File::open(blob.to_abs_path()).await?;
         let path_in_archive = PathBuf::from(BLOBS_BACKUP_NAME).join(blob.as_name());
         builder.append_file(path_in_archive, &mut file).await?;
+
+        // Appending a file does not flush it immediately,
+        // so it may get buffered and not sent over the network
+        // immediately.
+        // Documentation at
+        // <https://docs.rs/astral-tokio-tar/0.6.3/tokio_tar/struct.Builder.html#method.get_mut>
+        // suggests flushing the stream manually.
+        builder.get_mut().flush().await?;
     }
 
     builder.finish().await?;