RUBY-574 avoid doing special queries all the time
RUBY-572 fixing call to checkout_reader in DB#authenticate

The interface for MongoReplicaSetClient#checkout_reader was changed recently in RUBY-475. This usage of checkout_reader was missed and currently calls to DB#authenticate raise an exception.
clean-up: out-dated, we're not running them and auxiliary is misspelled \o/
RUBY-572 adding auth tests for replica sets and sharded clusters