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/blob_saver_test.go

@@ -54,8 +54,8 @@ func TestBlobSaver(t *testing.T) {
 	}
 
 	for i, blob := range results {
-		blob.Wait(ctx)
-		if blob.Known() {
+		sbr := blob.Take(ctx)
+		if sbr.known {
 			t.Errorf("blob %v is known, that should not be the case", i)
 		}
 	}