Parallelize data IO where possible

[j6k4m8]

• 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