[imurashka]

Saving no longer blocks the caller until the data is on disk. Changes are still serialized on the calling thread, and only the file write is handed over to a shared background thread, one storage at a time. A newer change replaces an unwritten older one, since the file is always written whole and only the last state matters.

Save() keeps its meaning and blocks until the data has actually reached the disk, and disposing a storage flushes whatever is still pending. Pass SaveOnBackgroundThread(false) to the builder to get the previous behaviour, where every change writes the file before returning.

The trade-off is that a change now reaches the disk a moment after the call returns instead of during it. Publication stays atomic, so a process killed inside that window keeps the previous generation intact and loses only the last change.