Refactor Schema/Metadata

As part of the pluggable storage engine effort, we'd like to make schema/metadata to be aware of storage engine concept, and be able to init storage engine instances for different keyspace/CFS.

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