KeyValueStore: use generic KeyValueDB::create()

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add superblock to KeyValueStore

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
FileStore: Add omap_backend to "<<" operator

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #2174 from yuyuyu101/kvstore-superblock

Kvstore superblock

Reviewed-by: Sage Weil <sage@redhat.com>