Abstract storage engine API from Keyspace/CFS

At this moment, we hope we already finished the refactor of most components in Cassandra, and we should be able to abstract a storage engine API, which defines a clear boundary for Cassandra storage engine.

For now, refer to https://docs.google.com/document/d/1suZlvhzgB6NIyBNpM9nxoHxz_Ri7qAm-UEO8v8AIFsc for high level designs.