ReplicationCoordinatorExternalState and data replication must not be started after shutdown

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.