Search: adjust query boosting, add recency filter, and minor refactoring

[gibrown]

Fixes #8590

• Adjusts what fields we boost on when running our queries (now I really want an all_content field)
• Adds separate groups of fields for boosting on. All of the boosting is very ad-hoc. Needs more data and testing rather than this hand tuned mess
• Fixes a bug where the default search query was not working if the site had a VIP index (requires adding define( 'JETPACK_SEARCH_VIP_INDEX', true ) as proposed in Search: max_offset is wrong in multiple ways #8560 )
• Added a filter for adjusting the recency scoring as discussed in Search: add filters to allow customization of the search algorithm #7975 cc @jeherve
• I looked at adding other filters, but feels like the other parts of the alg are likely to change too much.
• Cleaned up some code, and made some vars and functions protected

@mdbitz @nickdaugherty the VIP index bug was pretty big here. Also, I think we had broken any cases where query_fields was used to over ride the fields. If you have any real sites set up, looking at the actual query results quality with and without this would be good too.