Make test_idempotent run with ObjectStore

Add a new argument specify objectstore type

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Rename FileStoreDiff to ObjectStoreDiff

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Make test_idempotent_sequence run with ObjectStore

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>