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 maximum number of selected terms was equal to the
number of items times 'max_query_terms'. Instead, users should have direct control
over the maximum number of selected terms allowed, regardless of the number of
queried 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 across ALL items, not within each item. This could lead to
search results where the best matching items share commonalities amongst the
best characteristics of all the items.

Closes #6404