Consider adding support for update-in-place

[wardle]

For some installations, it might be useful to be able to update-in-place.

That means a server could run a cron job and update weekly automating maintenance.

The current model is that a data file is immutable once created. We would have to take care to ensure an update process did not result in temporarily broken concurrent read operations, but this is likely to be simple to check and fix if required.