increase work_mem for hivemind_app.update_notification_cache

(cherry picked from commit 335f21b1)

4d5612b8 increase work_mem allowed for update_notification_cache
a6708716 increase work_mem for get_discussion to avoid temp file creation
07e0a384 Revert "increase work_mem for get_discussion to avoid temp file creation"

Co-authored-by: Dan Notestein <dan@syncad.com>