[Snapshot] Implement Snapshot Delete CLI and API

Snapshot delete through a Ratis transaction.

The Ratis tx moves the snapshot from ACTIVE state to DELETED.

This does not remove the snapshot's RocksDB checkpoint directory. That would be done by a new background service SnapshotDeletingTask, which would also move the snapshot to RECLAIMED state and eventually remove the snapshotInfo from the table and remove the snapshot DB checkpoint.