Support multiple columns (composite key) for `merge_insert`

[westonpace]

Example scenario: A document is chunked into paragraphs and each paragraph is embedded and the row contains the document_id and the paragraph_id. Later, the user recalculates the embedding for one of the documents and wants to replace the rows.

[westonpace]

Adding merge_insert support isn't too bad. However, for performance reasons, we may also want to tackle #3125

[wjones127]

Looking at the code, this seems to work, but

1. Isn't well tested
2. Won't use any scalar indices

We can leave this issue open to implement those two.