This requires adding LFS as a dependency because native Lua has
no way to iterate a directory, but it seems to be Not Huge and
hopefully we'll have other uses for it
Previously: the service wrote a timestamp and the receiver
read and parsed it to see if there was new data
Now: the service writes and removes a .lock file to prevent
the receiver reading partial data. The receiver is responsible
for remembering the *previous* state and only updating if it's changed
Daniel Barlow