The PeriodicBalancerConfigRefresher puts its PeriodicJob on the ServiceContext's PeriodicRunner, and PeriodicJobs on the ServiceContext's PeriodicRunner can be modified directly by shutdown.

The PeriodicBalancerConfigRefresher is also modified on stepdown.

Since shutdown and stepdown can occur concurrently, and PeriodicJob has invariants that its methods are called in a specific order, either PeriodicBalancerConfigRefresher needs to add synchronization on top of its PeriodicJob, or PeriodicJob should have more lax invariants.