Improved hot-wiring of set_found_posts

[spacedmonkey]

Currently you can filter posts_request and posts_result to hot wiring the query object add caching. However, this means that in many cases the set found posts method then never gets called. This means that found posts is un-filterable. The set found posts needs to be called after the post results filter.

[boonebgorges]

Related: #36687

[pbearne]

will need some more tests