MGO-35 add support for ServiceHostname for GSSAPI, test for ServiceName

See corresponding code in server's MongoClient [here](https://github.com/mongodb/mongo/blob/be05259d36ec5f44c821c8f2d103a4ee83ebaf53/src/mongo/client/sasl_client_session.cpp#L279-285)
Let's please use the field name "ServiceHost", to follow usual Go conventions (the "host" term is abundant in Go code.. see the standard net package for example).