This is a multi-steps process:
* get a update's `user_id` from the metadata
* store the `user_id` (`lastUpdatedBy`) and current date (`lastUpdatedAt`) for
the document in Redis on every updates
* fetch `lastUpdatedAt` and `lastUpdatedBy` from Redis on document flush
* send the data to web to be persisted in Mongo
The version number is only used by the doc updater and so can be cleanly
encapsulated in a collection that only the doc updater knows about. The version
is already stored under docOps, so continue to tore it there.
