enable mongodb engine support mongodb replicaset client

django_mongodb_engine supports only MongoClient, but not MongoReplicaSetClient. This request replaced the defaut connection to MongoReplicaSetClient if any of read_preference, slave_okay, slaveok options was set in database conf.

And a warning was added if slave_okay, or slaveok was used in options, as both of them are deprecated settings in pymongo.