public synchronized int highestGen(String prefix) {
        int gen = 0;
        for (LogFile lf : knownFiles) {
            if (prefix.equals(lf.prefix)) {
                gen = Math.max(gen, lf.generation);
            }
        }
        return gen;
    }
Creates an instance of FilesArchive managing the given directory
logserver/src/main/java/com/yahoo/logserver/handlers/archive/FilesArchived.java