OM changes for tracking Snapshot Metadata

This Jira will be used to track changes for maintaining additional metadata to support Snapshot feature. This will include on-disk structure i.e. new RocksDB table(s) to track 

• all buckets that are snapshotted
• list of all the snapshots for such buckets 

In-memory structure would maintain an LRU cache of snapshot-chains for snapshotted buckets as required. - see HDDS-6983

Also, Refer to CheckpointBasedDesign doc for details.