Unexpectedly slow scalar index performance

[westonpace]

This was reported from Discord. When querying an indexed column with a very simple query (unique_id_col = some_id) there was a lot of CPU time spent in the materialize index node. It looks like we are always materializing the entire list of row IDs (this table had millions of rows) even if the allow list is very small. We can probably pick the smaller of the two lists to iterate on if there is no block list.