Move testing for known blobs to Archiver

This removes the list of in-flight blobs from the master index and
instead keeps a list of "known" blobs in the Archiver. "known" here
means: either already processed, or included in an index. This property
is tested atomically, when the blob is not in the list of "known" blobs,
it is added to the list and the caller is responsible to make this
happen (i.e. save the blob).

checker/repacker.go

@@ -136,7 +136,7 @@ func repackBlob(src, dst *repository.Repository, id backend.ID) error {
 		return errors.New("LoadBlob returned wrong data, len() doesn't match")
 	}
 
-	_, err = dst.SaveAndEncrypt(blob.Type, buf, &id, true)
+	_, err = dst.SaveAndEncrypt(blob.Type, buf, &id)
 	if err != nil {
 		return err
 	}