BTree index training on string column uses lots of memory

[wjones127]

AC: have a test that shows that BTree index can run while using a limited amount of memory

[westonpace]

We disable spilling automatically on string columns. However, the Datafusion bug around string spilling was recently closed so maybe we can remove this limitation once we upgrade.

[wjones127]

Upstream issue: apache/datafusion#10073

Fixed by: apache/datafusion#14644

This fix should be available in DataFusion 46.

We currently bypass spilling for string, so need to remove the bypass.