static List<Trigger> parseReady(ReindexingConfig.Clusters cluster, DocumentTypeManager manager) {
        return cluster.documentTypes().entrySet().stream()
                      .map(typeStatus -> new Trigger(manager.getDocumentType(typeStatus.getKey()),
                                                     Instant.ofEpochMilli(typeStatus.getValue().readyAtMillis()),
                                                     typeStatus.getValue().speed()))
                      .toList();
    }
Runs in all cluster controller containers, and progresses reindexing efforts.
Work is only done by one container at a time, by requiring a shared ZooKeeper lock to be held while visiting.
Whichever maintainer gets the lock holds it until all reindexing is done, or until shutdown.

@author jonmv
clustercontroller-reindexer/src/main/java/ai/vespa/reindexing/ReindexingMaintainer.java