Refactor sendAuthRequest.

This way sendAuthRequest doesn't need to know the details of all the
different authentication methods. This is in preparation for adding SCRAM
authentication, which will add yet another authentication request message
type, with different payload.

Reviewed-By: Michael Paquier
Discussion: <CAB7nPqQvO4sxLFeS9D+NM3wpy08ieZdAj_6e117MQHZAfxBFsg@mail.gmail.com>