Auto-record data on document create / update

@wohali:
Think: timestamp, CouchDB username. Would be added by the coordinating node in the cluster.

@akircher:
Could auto-recording timestamp and username be solved more generically? Such as a "calculated field" function. This function could return a timestamp, username, or anything else that could be calculated from the doc (and maybe the req) object. I know this would be very helpful with my current project.

@wohali:
That is doable today via update handlers, and necessarily requires a JS-based implementation, which is not always desirable for performance reasons.

The discussion identifies document create/update handling and existing JavaScript update handlers as the starting points. Review those extension points, then clarify whether the goal is automatic timestamp and username recording or a generic calculated-field function before implementation.

Written by the indexing model from the issue text.