Honestly that's not something I would handle at the db layer. I would build a service that is responsible for keeping client db's in sync and abstract that away from my database entirely.
Interesting. Tbh. I'm not looking for solutions at the moment, I just know that back when we decided on the technology, CouchDB was pretty much the only good player in town. I was just curious what's out there today. RethinkDB is being mentioned a lot, I'll have to check it out some time.