get_pages: add query hooks, wpcache argument hooks

[kevinB]

To support plugins which want to do query filtering and/or use alternate wp_cache id and group (Note that the existing results filter already lets plugins have their way with results set. This just lets them do it more efficiently).

• Add wpcache_id and wpcache_group filters

• Add join, where, request filters

• apply existing results filter prior to cache update

[kevinB]

also posted tickets for get_bookmarks(#4095) and get_categories(#4096)

[kevinB]

correction: the existing results filter is already applied prior to cache update. Tickets #4095 and #4096 propose changing get_bookmarks and get_categories to do the same.

[kevinB]

(updated: moved results filter to the end)

[kevinB]

I've come to some clearer thinking on the results filter placement.

If the cache retrieval is run through the same filter, there is no need to filter prior to cache storage. It seems safer to leave the actual cache storage unfiltered so it's not invalid after a filtering plugin is removed.

That means get_pages is the one that should change.

In fact, there is a defect in the current code:
The current placement of the post-query "get_pages" filter causes get_page_children contributions to be filtered on cache retrieval, but not after query. End user could get a different answer the first time they ask.