This is done by/for kosync plugin at each opening, because
the docsettings was re-opened and saved for this, but later
overwritten by the current koreader docsettings - so it was
redone each time. This correctly adds this partial_md5_checksum
to the current koreader docsettings, which will be saved on
document closing - so it will not be redone next time.
Note: this partial_md5_checksum is not (yet) used by anything.