When we change classes in our webapps that uses persistent sessions, the older sessions throw 
NotSerializableExceptions (because the class changed).   This causes a couple of problems.  First, 
the thread that checks for timeouts (PersistentManagerBase.run()) exits.   Second, if we try to get a 
session that contains objects of the old (unserializable) class then it returns a "valid" session with 
nothing in it (shouldn't it be invalidated?).