merge doc fix and perf fix/warning to session_dbm.py:clean()

Author: rouilj

doc/customizing.txt

@@ -1892,7 +1892,7 @@ To edit the status of all items in the item index view, edit the
    and at the bottom of that table add::
 
      </table>
-    </form
+    </form>
 
    making sure you match the ``</table>`` from the list table, not the
    navigation table or the subsequent form table.

roundup/backends/sessions_dbm.py

@@ -203,15 +203,18 @@ def clean(self):
         ''' Remove session records that haven't been used for a week. '''
         now = time.time()
         week = 60*60*24*7
+        a_week_ago = now - week
         for sessid in self.list():
             sess = self.get(sessid, '__timestamp', None)
             if sess is None:
                 self.updateTimestamp(sessid)
                 continue
-            interval = now - sess
-            if interval > week:
+            if a_week_ago > sess:
                 self.destroy(sessid)
 
+        run_time = time.time() - now
+        if run_time > 3:
+            self.log_warning("clean() took %.2fs", run_time)
 
 class Sessions(BasicDatabase):
     name = 'sessions'