public synchronized void close() {
        Iterator<LogWriter> it = logWriterLRUCache.values().iterator();
        while (it.hasNext()) {
            LogWriter l = it.next();
            try {
                l.close();
            } catch (IOException e) {
                log.log(Level.WARNING, "Closing failed", e);
            }
            it.remove();
        }
    }
Generate prefix for log filenames based on log message.
<p>
<EM>This message is <code>public</code> only because we need
access to it in unit tests.  For all practical purposes this
method does not exist to you, the application programmer, OK? :-)</EM>
<p>
XXX optimize!
logserver/src/main/java/com/yahoo/logserver/handlers/archive/ArchiverHandler.java