Auto-extend session lifetime in DiskStore

If you use DiskStore for sessions (provided they are checked against expiration), they expire 'timeout' after the session was created, not last accessed (which is the behaviour of DBStore). So I added os.utime() [a.k.a. "python touch"] as similar operation to what is in DBStore:

self.db.update(self.table, where="session_id=$key", atime=now, vars=locals())

I hope this commit makes the two storages more similar in behaviour.