Instead of keeping track of last normalization (processing) date, we now keep track of
an internal processing version that will help us understand what kind of processing has
already been completed for a given attachment. This will let us retroactively upgrade
existing attachments.
As we add more processing steps, we can build a processing pipeline that can convert any
attachment processing version into a higher one, e.g. 4 -> 5 -> 6 -> 7.
