It is possible for the ReplicationCoordinatorImpl to call startThreads on the ReplicationCoordinatorExternalState after calling shutdown() on the external state. In this case the ReplicationCoordinatorExternalState should refuse to start.

Additionally, ReplicationCoordinatorImpl::_startDataReplication may be called after shutdown().
This calls _externalState->startSteadyStateReplication, which should also do nothing after shutdown.

Further, if initial sync is required, ReplicationCoordinatorImpl::_startDataReplication should avoid creating the initial syncer after shutdown.