Enhancement: Full sync query is slow on larger sites

### Impacted plugin

Boost

### What

The full sync query could be optimized or cached

### How

The full sync that seems to run on shutdown on some pages can be slow on larger sites. It may be a larger than average post table or a large number of post types that causes this.

This issue is reproducible on the WCCOM staging site on the WC status tools page.

Request URL: `/wp-admin/admin.php?page=wc-status&tab=tools`
Average query time: ~3.5s

Start at the WooCommerce status tools entry point `/wp-admin/admin.php?page=wc-status&tab=tools` on the WCCOM staging site and inspect the Boost full-sync work that runs during shutdown. Profile the full-sync query against a larger site, then compare its average time with the reported ~3.5 seconds. Done means the full sync is measurably faster without changing its results.

Written by the indexing model from the issue text.