Remove SequencerPosition from KeyValueStore

Now KeyValueStore expects kv backend to ensure consistency and there is unusable
for KeyValueStore to store a SequencerPosition.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Fix write operation on a deleted object in the same transaction

If the following op happened:
touch obj
delete obj
write obj

KeyValueStore will fail at "write" operation.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add test for objectstore

Add remove then write object in a transaction.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1941 from yuyuyu101/wip-8564

Fix write operation on a deleted object in the same transaction(KeyValueStore)

Reviewed-by: Sage Weil <sage@inktank.com>