Services - Detect record write collisions

[dvstans]

Currently, if multiple people edit the same record, the last person's update will override all other changes, and no one will realize the conflict occurred. The "rev" field can be used to detect collisions before they happen inside the database. This requires the rev field to be returned in all "get" methods, and also passed back to the DB for all "update" methods. If the revs don't match on update, this indicates a collision and the app can take appropriate action (get the latest version, diff, and allow user to decide how to proceed).