'Callback to handle the start of a hash
`key` is the redis key
`length` is the number of elements in this hash.
`expiry` is a `datetime` object. None means the object does not expire
`info` is a dictionary containing additional information about this object.
After `start_hash`, the method `hset` will be called with this `key` exactly `length` times.
After that, the `end_hash` method will be called.'

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

    pass
