'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):

    if ((self.object_id >= (10 ** 10)) or (self.verb.id >= (10 ** 3))):
        raise TypeError('Fatal:    object_id    /    verb    have    too    many    digits    !')
    if (not self.time):
        raise TypeError('Cant    serialize    activities    without    a    time')
    milliseconds = str(int((datetime_to_epoch(self.time) * 1000)))
    serialization_id_str = ('%s%0.10d%0.3d' % (milliseconds, self.object_id, self.verb.id))
    serialization_id = int(serialization_id_str)
    return serialization_id
