RocksDB based storage engine

As I mentioned in CASSANDRA-13474, we got huge P99 read latency gain from the RocksDB integration experiment.

After we make the existing storage engine to be pluggable, we want to implement a RocksDB based storage engine, which can support existing Cassandra data model, and provide better and more predictable performance.

The effort will include but not limited to:
1. Wide column support on RocksDB
2. Streaming support on RocksDB
3. RocksDB instances management
4. Nodetool support
5. Metrics and monitoring
6. Counter support on RocksDB