Rewrite CassandraModel#stream_all_indexed_slices

It might be a good idea for @ryanwilliamson or @blanquer to take a look at this rewrite. It's less efficient, but our old scheme was fatally buggy vs. Cassandra 1.1 (which now sends the index column OUT OF ORDER when you get_indexed_slices, thereby ruining our column-iterator logic) and it was also double-yielding the index column sometimes.