enum PrimaryTransitionState {
Secondary,
CatchingUp,
WaitingForDrainToBecomePrimary,
WaitingForDrainButStaySecondary,
Primary,
}

If a stepdown occurs in CatchingUp, we'd go straight back to Secondary. If it
happens in WaitingForDrainToBecomePrimary, we'd go to
WaitingForDrainButStaySecondary.