Additional optimisation for SASL / JAAS integration when non-JAAS realm used for authentication

The JAAS based authentication and related caching makes an assumption that a username and password is available for the authentication process - where a user has already been authenticated using a SASL mechanisms this is most likely not the case e.g. Digest, Kerberos...

To bridge this gap we create a fake username and password before the JAAS call - this task is to optimise that so that if a connection is held open the same generated username and password should be used so that any cached value can be used without triggering a JAAS auth for each call.