zodb: go from undo id to TID

>>> conndb.undoInfo()[17]
{'description': '/www/fg_base_view_p3', 'size': 56700, 'user_name': ' None', 'id': 'A6kMYt2AfSI=', 'time': 1408675371.9145098}
# now it's time to transfor the undo id to a TID
>>> import base64
>>> base64.decodestring('A6kMYt2AfSI=' + '\n')
'\x03\xa9\x0cb\xdd\x80}"'
>>> u64(base64.decodestring('A6kMYt2AfSI=' + '\n'))
263755671940398370