Make SessionDataManagerTraverser inherit from Persistent. This is in order to...

Make SessionDataManagerTraverser inherit from Persistent.  This is in order to avoid a situation in which a nonpersistent instance of this class is inserted into the before traverse hook, causing Zope to not be able to start if the class' containing module can't be imported.  The existence of the bug stemmed from my lack of knowledge about how the traverse hook worked (I didn't know it actually stored an object reference).

(merge from branch)

lib/python/Products/Sessions/SessionDataManager.py

@@ -234,7 +234,7 @@ class SessionDataManager(Item, Implicit, Persistent, RoleManager, Owned, Tabs):
             self._hasTraversalHook = 1
             self._requestSessionName = requestSessionName
 
-class SessionDataManagerTraverser:
+class SessionDataManagerTraverser(Persistent):
     def __init__(self, requestSessionName, sdm):
         self._requestSessionName = requestSessionName
         self._sessionDataManager = sdm