archiver: Asynchronously complete FutureFile

After reading and chunking all data in a file, the FutureFile still has
to wait until the FutureBlobs are completed. This was done synchronously
which results in blocking the file saver and prevents the next file from
being read.

By replacing the FutureBlob with a callback, it becomes possible to
complete the FutureFile asynchronously.

internal/archiver/file_saver_test.go

@@ -34,10 +34,8 @@ func createTestFiles(t testing.TB, num int) (files []string, cleanup func()) {
 func startFileSaver(ctx context.Context, t testing.TB) (*FileSaver, context.Context, *errgroup.Group) {
 	wg, ctx := errgroup.WithContext(ctx)
 
-	saveBlob := func(ctx context.Context, tpe restic.BlobType, buf *Buffer) FutureBlob {
-		ch := make(chan SaveBlobResponse)
-		close(ch)
-		return FutureBlob{ch: ch}
+	saveBlob := func(ctx context.Context, tpe restic.BlobType, buf *Buffer, cb func(SaveBlobResponse)) {
+		cb(SaveBlobResponse{})
 	}
 
 	workers := uint(runtime.NumCPU())