Server-side mass update function

We get this one a lot on IRC: "I want to write a JS function that goes to CouchDB and CouchDB runs it on every document in the database." Basically, a way to submit server-side jobs to update all the docs in the database somehow.

The issue names no files, tests, or existing entry points. Start by reviewing CouchDB's server-side JavaScript and database update mechanisms, then clarify the API, execution and safety requirements; done would require an agreed design and tests showing updates can be applied across all documents.

Written by the indexing model from the issue text.