More Like This Query: creates only one MLT query per field for all the items.

Previously, one MLT query per field was created for each item. One issue with
this method is that the number of total selected terms per field is equal to the
number of items times 'max_query_terms'. Instead, 'max_query_terms' should
directly control the total maximum number of terms allowed regardless of the
number of items.

Another issue related to the previous method is that it could
lead to the selection of rather uninteresting terms, that because they were
found in a particular queried item. Instead, this new procedure enforces the
selection of interesting terms amongst ALL items, leading to search results
where the best matching items share commonalities amongst each item.