Code review comment for lp:~robertcarr/mir/implement-client-credentials

Alan: Issues addressed, mostly leftovers from the first version of the branch (i.e. ::invoke variant is just deleted).

Gerry: I'm having some difficulty figuring out how this session name setting bit should work. So I think in the mean time, we can progress with this, and the limitation that the app only allows one app to be in the 'launching' state at a time (to prevent authentication races).