archiver: reduce memory usage for large files

FutureBlob now uses a Take() method as a more memory-efficient way to
retrieve the futures result. In addition, futures are now collected
while saving the file. As only a limited number of blobs can be queued
for uploading, for a large file nearly all FutureBlobs already have
their result ready, such that the FutureBlob object just consumes
memory.

internal/archiver/file_saver_test.go

@@ -34,7 +34,7 @@ func startFileSaver(ctx context.Context, t testing.TB) (*FileSaver, context.Cont
 	wg, ctx := errgroup.WithContext(ctx)
 
 	saveBlob := func(ctx context.Context, tpe restic.BlobType, buf *Buffer) FutureBlob {
-		ch := make(chan saveBlobResponse)
+		ch := make(chan SaveBlobResponse)
 		close(ch)
 		return FutureBlob{ch: ch}
 	}