More Like This Query: creates only one MLT query per field for all queried 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 queried item.