C++ driver should accept both "userSource" and "db" when specifying what DB to authenticate against in the auth helper

Everywhere else we refer to a user's "home" database we have changed the word "source"/"userSource" to "db". But the C++ driver's auth method still takes just "userSource" as the way to specify what db to run the authenticate command against.

Since this behavior was already released in 2.4, we have to continue supporting userSource at least temporarily, but we should introduce "db" as an alias and start using that in all places in our code where we call the C++ driver's auth helper.