Read-ledger without storage copy in case of RocksDB (RocksDB read-only mode support)

Unlike LevelDB the RocksDB supports read-only mode. Several processes can open the same storage in read-only mode. It means that it is not necessary to create storage copy by read_ledger script to read all entities. Instead the storage can be opened in read-only mode by read_ledger script if it is a rocksdb storage, but for now read-only option is not supported by our code.