@Override
    protected double maintain() {
        applicationRepository.deleteUnusedFileDistributionReferences(fileDirectory);
        return 1.0;
    }
Removes unused file references older than a configured time, but always keeps a certain number of file references
even when they are unused.
<p>
Note: Unit test is in ApplicationRepositoryTest

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