alarms: fix natural order comparator to use timestamp first

Motivation:

The natural order sorting of alarms currently is
based on its key.  This will work for most types
of alarms because the timestamp is included in
the key string.  However, generic alarms
have a UUID as key, so sorting is suboptimal
in that case.

Modification:

Sort first on the last modified timestamp.  Should
those be equal, then try creation time, and only
thereafter use the key to sort them.

Result:

All alarms are implicitly ordered by at least
their latest modification timestamp.

Target: master
Request: 3.2
Request: 3.1
Request: 3.0
Request: 2.16
Require-notes: no
Require-book: no
Acked-by: Paul

modules/dcache/src/main/java/org/dcache/alarms/LogEntry.java

@@ -60,6 +60,7 @@ LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 package org.dcache.alarms;
 
 import com.google.common.base.Preconditions;
+import com.google.common.collect.ComparisonChain;
 
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
@@ -106,8 +107,14 @@ private static String getFormattedDate(Date date) {
 
     @Override
     public int compareTo(LogEntry o) {
-        Preconditions.checkNotNull(o, "alarm entry parameter was null");
-        return key.compareTo(o.key);
+        if (o == null) {
+            return -1;
+        }
+        return ComparisonChain.start()
+                              .compare(lastUpdate, o.lastUpdate)
+                              .compare(firstArrived, o.firstArrived)
+                              .compare(key, o.key)
+                              .result();
     }
 
     @Override