Refactor compaction

As part of the pluggable storage engine effort, we'd like to refactor compaction, move it into existing storage engine implementation details, and make keyspace/columnfamilystore not leak compaction details everywhere.

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