Refactor RocksDBCheckpointDiffer class

Currently, `RocksDBCheckpointDiffer` does lots of things (e.g. maintaining the DAG, implements RocksDB listener, diff two snapshots and probably remove/update of snapshot from DAG too) which makes it complicated and harder to test because of tight coupling with RocksDB.
`RocksDbCheckpointDiffer` can be simplified by extracting out the DAG to new class `RocksDbCompactionDag` which maintains SST DAG in which you can add nodes & arcs, load the DAG from disk/DB, remove nodes and arcs, etc. DAG is independent of what we use to store compaction log (disk or RocksDB). We should be able to test all the functionality of `RocksDbCompactionDag` independently.