public Optional<Instant> getDeadline() {
        return Optional.empty();
    }
If the task completion has been deferred due to hasVersionAckDependency(),
 this method will be invoked if a failure occurs before the version has
 been successfully ACKed.

 LEADERSHIP_LOST will be the failure condition if the cluster controller
 discovers it has lost leadership in the time between task execution and
 deferred completion time.

 DEADLINE_EXCEEDED will be the failure condition if the completion has been
 deferred for more than a configurable amount of time.

 This method will also be invoked if the controller is signalled to shut down
 before the dependent cluster version has been published.

 The task implementation is responsible for communicating the appropriate
 error semantics to the caller who initially scheduled the task. If additional
 details are available, Failure.getMessage() will return a non-empty string.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/RemoteClusterControllerTask.java