'serialization_id is used to keep items locally sorted and unique
(eg. used redis sorted sets\' score or cassandra column names)
serialization_id is also used to select random activities from the feed
(eg. remove activities from feeds must be fast operation)
for this reason the serialization_id should be unique and not change over time
eg:
activity.serialization_id = 1373266755000000000042008
1373266755000 activity creation time as epoch with millisecond resolution
0000000000042 activity left padded object_id (10 digits)
008 left padded activity verb id (3 digits)
:returns: int --the serialization id'

@property
def serialization_id(self):

    milliseconds = str((int(datetime_to_epoch(self.updated_at)) * 1000))
    return milliseconds
