[webrunner]: use newest timestamp in equeue.db to get last backup date

in some webrunner, there are 2 keys in equeue.db:

```
>>> print(db.keys())
['/srv/slapgrid/slappart18/bin//runner-importer', '/srv/slapgrid/slappart18/bin/runner-importer']
```

As you can see, the importer script path changed. But we don't care
about this, we always want to have the latest (= the newest) backup date.

stack/resilient/resilient-web-takeover-cgi-script.py.in

@@ -41,17 +41,17 @@ def getLatestBackupDate():
   equeue_database_copy = os.path.join(temporary_directory, 'equeue.db')
   shutil.copyfile(equeue_database, equeue_database_copy)
   db = gdbm.open(equeue_database_copy)
-  # Usually, there is only one callback (so only one key
-  # in the db), but if there are several:
-  # Take the "oldest" one (oldest value).
+  # Usually, there is only one callback (so only one key in the db), but if
+  # there are several we take the "newest" one.  Indeed, sometimes the importer
+  # script change name those introducing a new key inside the db.
   db_keys = db.keys()
   if not db_keys:
     result = False
   else:
-    last_backup = db[db_keys[-1]]
+    last_backup = db[db_keys[0]]
     for callback in db_keys:
       timestamp = float(db[callback])
-      if timestamp < last_backup:
+      if timestamp > last_backup:
         last_backup = timestamp
     result = datetime.datetime.fromtimestamp(last_backup)
   db.close()