move

Move all storage to newSavePath, the counterpart of libtorrent's async_move_storage (storage_utils.cpp move_storage). Open handles are flushed and closed, the on-disk files (and any part-file) are relocated under the new save path, and subsequent reads/writes target the new location. A best-effort, no-op default is provided for backends without on-disk state (e.g. the in-memory double).