Stop using switch_to_blog()

For large/high-traffic sites, switch_to_blog() just isn't going to cut the mustard. We need Another Way.

My immediate thought is to use custom WP-API endpoints, but that relies on WP-API being adopted into core which will happen.... one day.... far in the future.... maybe.

Start by locating the code paths that call switch_to_blog() and review how the aggregator synchronises posts across the multisite network. Investigate whether custom WP-API endpoints are available or suitable, and define a replacement approach that supports large, high-traffic sites without relying on switch_to_blog().

Written by the indexing model from the issue text.