public Optional<ApplicationInfo> getApplicationInfo(DomainName hostName) {
        return Optional.ofNullable(idsByHostname.get(hostName)).map(applicationsById::get);
    }
A non-thread-safe mutable container of ApplicationInfo, also taking care of listeners on changes.

@author hakonhall
service-monitor/src/main/java/com/yahoo/vespa/service/duper/DuperModel.java