fdbcli snapshot command does not work with RocksDB

The fdbcli snapshot command takes a filesystem snapshot as a way to backup FDB by copying the files on disk. The snapshot is not atomic across multiple files, so the snapshot can end up with corrupt RocksDB files due to compaction. The RocksDB snapshot command should be used instead.

Start by locating the fdbcli snapshot command and the code path that handles RocksDB. Compare its current filesystem-copy behavior with the intended RocksDB snapshot command, then verify that snapshots remain usable during compaction and that the relevant command behavior is covered by tests.

Written by the indexing model from the issue text.