(#926) - Set _rev when updating checkpoint docs

_local docs, used to store checkpoints (from which point to start when
replicating), are treated differently on CouchDB and on BigCouch
(Cloudant). The first enforces _rev checking for updates, the later
couldn't care less about _rev. This produced some drama in the past,
causing developers to make Pouch remove _rev attributes from checkpoint
docs because of BigCouch, but know everything appears to be ok if we
just send them, because both BigCouch and CouchDB will accept them.