JAVA-1332 with minimal Async changes.

I removed the larger async changes from #19 as it seemed to get in the way of the review and the updating of async/rx is best done once the apis are solidified.
I think this is now too limiting, since it no longer lets you move the collection to a different database.  What about the idea of moving this method to mongoCollection, where the "old" name would be implicit, and using MongoNamespace for the new collection name?