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. Read/write operations will be reduced by metadata
3. One object is constructed by multi key-value store objects
4. etc..

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