Core: fix index throttling to notice a change to max_merge_count on the next merge start/finish

Previously such changes were never noticed, and index throttling kept
kicking in at the original setting from startup.

Closes #8136

Closes #8132
Core: cutover to Lucene's query rescorer

This is functionally equivalent to before, so there should be no
user-visible impact, except I added a NOTE in the docs warning about
the interaction of pagination and rescoring.

Closes #6232

Closes #7707