'Callback to handle the start of a sorted set
`key` is the redis key for this sorted
`length` is the number of elements in this sorted set
`expiry` is a `datetime` object. None means the object does not expire
`info` is a dictionary containing additional information about this object.
After `start_sorted_set`, the method `zadd` will be called with `key` exactly `length` times.
Also, `zadd` will be called in a sorted order, so as to preserve the ordering of this sorted set.
After that, the `end_sorted_set` method will be called to indicate the end of this sorted set
Note : This callback handles sorted sets in that are stored as ziplists or skiplists'

def start_sorted_set(self, key, length, expiry, info):

    pass
