Add LevelDB support to OSD backend

This is the first draft for LevelDB support. Simply
implement necessary methods via LevelDB API.

The following roadmap is go:
1. Support multi thread
2. Unify OMap API
3. Read/write operations will be reduced by metadata
4. One object is constructed by multi key-value store objects
5. etc..

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