private void maintain() {
        for (Reindexer reindexer : reindexers)
            executor.submit(() -> {
                try {
                    reindexer.reindex();
                }
                catch (ReindexingLockException e) {
                    log.log(FINE, "Failed to acquire reindexing lock");
                }
                catch (Exception e) {
                    log.log(WARNING, "Exception when reindexing", e);
                }
            });
    }
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