FixingRoleDecider(ref: StateRef, progressTracker: ProgressTracker = tracker())
This protocol looks at the deal and decides whether to be the Fixer or Floater role in agreeing a fixing.
It is kicked off as an activity on both participant nodes by the scheduler when its time for a fixing. If the Fixer role is chosen, then that will be initiated by the FixingSession message sent from the other party and handled by the FixingSessionInitiationHandler.
TODO: Replace FixingSession and FixingSessionInitiationHandler with generic session initiation logic once it exists.