Use a table waiter rather than sleep

[mrichman]

Table waiters are more deterministic than sleep().

https://github.com/trek10inc/ddb-single-table-example/blob/d54358a441c7305bd9d5ebbd1f2876e5a53f44a9/load.py#L57

waiter = dynamodb.meta.client.get_waiter(tablename)
waiter.wait(TableName=table_name)