Add RocksDB Storage

After S2GRAPH-17, It becomes easy to add storage backend. I am suggesting to add RocksDB Storage.

the purpose is following.

1. RocksDB has great performance on single machine. even though our current default backend `HBase` is great for production environment, sometimes setting up HBase cluster is requiring lots of work. many of users just want to try with small graph and single machine implementation can be helpful for development and just trying out phase.

2. for production use of RocksDB, I think both of sharing management and replication are needed to be solved. personally This is fascinating topics to play around and I think our code base is nice place to study and apply these.

I am not expecting we can finish 2 in any time soon(maybe seperate issue need to be open), but I want to keep studying and discuss this topic even though it will takes time.

For this Issue I think phase 1 is very easy to implement but phase 2 is hard and taking lots of effort and time, so I am thinking about finish phase 1 as soon as possible and see what we can do for phase 2.

Feel free to join me on this issue and any feedback would be appreciated.