MDL-32462 accesslib: fix context_user::build_paths performance
MDL-32462 accesslib: fix context_user::build_paths performance

The problem was that the query was updating every row whether it needed
it or not. This turns out to be a really bad thing to do on Postgres,
because it then triggers a long expensive vacuum, which lock the context
table for a long time a really bad thing to do.

dilawar/moodle