Fix non-intuitive repository behavior

- The SaveBlob method now checks for duplicates.
- Moves handling of pending blobs to MasterIndex.
  -> also cleans up pending index entries when they are saved in the index
  -> when using SaveBlob no need to care about index any longer
- Always check for full index and save it when storing packs.
  -> removes the need of an index uploader
  -> also removes the verbose "uploaded intermediate index" messages
- The Flush method now also saves the index
- Fix race condition when checking and saving full/non-finalized indexes

internal/repository/index_test.go

@@ -123,9 +123,10 @@ func TestIndexSerialize(t *testing.T) {
 		}
 	}
 
-	// serialize idx, unserialize to idx3
+	// finalize; serialize idx, unserialize to idx3
+	idx.Finalize()
 	wr3 := bytes.NewBuffer(nil)
-	err = idx.Finalize(wr3)
+	err = idx.Encode(wr3)
 	rtest.OK(t, err)
 
 	rtest.Assert(t, idx.Final(),