LocalStore no longer blocks the event loop on filesystem calls. open,
clear, delete, delete_dir, list, list_prefix, list_dir,
move and getsize now run their disk I/O in a worker thread via
asyncio.to_thread, as get and set already did, so other tasks
sharing the loop are not stalled while the store walks or modifies a directory
tree. The LocalStore test suite now fails if any async store method touches
the filesystem from the event loop thread.