Fixes: #7172

To ensure LevelDBStore::db is destroyed before dependent LevelDBStore::db_cache and LevelDBStore::filterpolicy.

Signed-off-by: Signed-off-by: Ray Lv <xiangyulv@gmail.com>
Merge pull request #1107 from xylv/wip-osd-crash-with-leveldb-cache-set

osd: OSD failed to start with osd_leveldb_cache_size > 0

Reviewed-by: Sage Weil <sage@inktank.com>