public void shutdown() {
        maintainers.forEach(Maintainer::shutdown);
        maintainers.forEach(Maintainer::awaitShutdown);
    }
Maintenance jobs of the config server.
Each maintenance job is a singleton instance of its implementing class, created and owned by this,
and running its own dedicated thread.

@author hmusum
configserver/src/main/java/com/yahoo/vespa/config/server/maintenance/ConfigServerMaintenance.java