Refactor Auth to exclusively use a MongoCredential object

According to the auth spec, we should be using an object called MongoCredentials to pass around all of our auth configuration. Right now, we have a variety of ways we pass around credentials, including:

• Sticking things together in an anonymous auth object
• Sticking things in an auth array
• Just passing fields around individually

By switching to tracking credentials in a single way, it would simplify a lot of code.