Use CF comparator to sort indexed columns in SecondaryIndexManager

SecondaryIndexManager is supposed to have it's internal map sorted according to the base CF comparator, but instead it sorts using the byte buffer natural ordering.

This order is carried along by the sorted set returned by getIndexedColumns(), which in turns end up in a NamesQueryFilter when reading indexed columns, so the order should really be the CF one.

I'll note that I don't think this is a bug because SSTableNamesIterator don't in fact rely on the actual ordering of the names. But it's worth fixing to avoid future problems.