'Create new shard state.
Args:
mapreduce_id: unique mapreduce id as string.
shard_number: shard number for which to create shard state.
Returns:
new instance of ShardState ready to put into datastore.'

@classmethod
def create_new(cls, mapreduce_id, shard_number):

    shard_id = cls.shard_id_from_number(mapreduce_id, shard_number)
    state = cls(key_name=shard_id, mapreduce_id=mapreduce_id)
    return state
