Parallelize data IO where possible

* [ ] `FilesystemStorageManager#getdata`:
https://github.com/aplbrain/bossphorus/blob/ef5caac373593ae934f12c14c4b5cd08152b0283/bossphorus/StorageManager.py#L125-L142
* [ ] `FilesystemStorageManager#setdata`:
https://github.com/aplbrain/bossphorus/blob/ef5caac373593ae934f12c14c4b5cd08152b0283/bossphorus/StorageManager.py#L82-L97